documentation: restructure into sections mirroring the toolbox

[philipcardiff]

Phase 1 of the /documentation/ restructure. The flat list of eight pages
becomes a tree of sections whose shape follows the toolbox, and roughly 1400
lines of documentation that were already committed upstream but reachable from
no page here are published.

Important

⚠️ Do not merge this PR as it stands

The submodule pointer here is 7410d74, the tip of the docs-phase1 branch
of solids4foam/solids4foam#324. That commit is not on development, so
merging now would point master at a commit that may never exist there and
the published site would break.

Note that update-submodule.yml in the toolbox repo pushes a submodule bump
straight to master here on every push to development. So once #324
merges, master gets the correct pointer on its own, and this branch will
conflict with it on imported/solids4foam.

Merge checklist, in this order:

• 1. Merge Documentation: surface the existing READMEs on the website solids4foam#324 into development. It adds the
  sort: front matter this PR's ordering depends on. Merge
  Documentation: normalise markdown tables to a consistent compact style solids4foam#326 too: it fixes the malformed
  viscoTube image link referenced below.
• 2. Wait for the "Automated update of submodule to the latest
  development commit" push to land on master here.
• 3. Merge master into this branch, resolving imported/solids4foam
  in favour of master:
  bash git checkout docs-phase1 git fetch origin && git merge origin/master git checkout --theirs imported/solids4foam # take master's pointer git submodule update --init git commit && git push
• 4. Confirm git submodule status no longer shows 7410d74, and that
  find documentation tutorials -type l ! -exec test -e {} \; -print
  prints nothing.
• 5. Re-run bin/sync-docs.sh and commit anything it changes. The
  new submodule pointer restores the viscoTube image that the
  manifest currently cannot link.
• 6. Mark ready for review (gh pr ready 82) and merge.

Structure

documentation/
├── overview.md
├── solid-models/          README + linGeomTotalDispSolid,
│                          nonLinGeomTotalLagTotalDispSolid,
│                          nonLinGeomUpdatedLagSolid
├── fluid-models/
├── fluid-solid-interfaces/
├── function-objects/
├── applications/          README + solids4Foam, utilities,
│                          decomposeParMonolithic, scripts
└── under-the-hood/        README + stabilisation-models, contributing

Content is imported exactly as before: git symlinks into
imported/solids4foam, with each page's images symlinked beside it.

fluid-models, fluid-solid-interfaces and function-objects become
directories too. The theme lists a directory's files before its
subdirectories and renders subdirectory links unnumbered, so mixing plain
pages with sections would have put the solid mechanics sections below the
fluid ones whatever their sort values.

URLs

Every previous URL still works. jekyll-redirect-from is added to the plugin
list and a site-owned stub left at each old path, so
/documentation/solid-models.html, utilities.html, scripts.html,
fluid-models.html, fluid-solid-interfaces.html and
function-objects.html redirect rather than 404. Redirect pages are filtered
out of the sidebar in site_pages.liquid.

Also

• overview.md trimmed to the ethos, an architecture map linking into the new
  sections, and the toolbox structure. The solids4Foam solver walkthrough
  moves upstream, to /documentation/applications/solids4Foam.html. The four
  stale solids4foam.github.io links are gone.
• tutorials/more-tutorials/fluid-mechanics/README.md added — it was the one
  tutorial section without an index page, so the sidebar printed its raw URL.
• Ten unreferenced images removed from documentation/images; the five still
  in use moved into the section directories that consume them.
• Typo in the solid mechanics tutorials landing page.

Verification

• jekyll build --safe against the companion branch: all 22 documentation
  pages render, and --safe does follow the symlinks.
• Whole-site link check over the built output: 118 pages, zero broken
  internal links (there were three before).
• No dangling symlinks under documentation/ or tutorials/.
• markdownlint-cli2 --config .markdownlint.json over documentation/**:
  clean. prettier --check on every file touched: clean.
• Each redirect stub verified to emit the intended target.

🤖 Generated with Claude Code

---

Update: symlink tooling (Phase 0 remainder)

Commit f12feec adds the manifest-driven tooling the plan's Phase 0 called
for. It lives here rather than in a separate PR off master because
bin/docs-manifest.txt describes the section tree this PR creates, so it
cannot stand alone.

• bin/docs-manifest.txt — one line per published page, mapping the site
  path to the path within the submodule. 71 entries.
• bin/sync-docs.sh — generates the page symlinks and each page's
  images/ directory from the manifest. Refuses to overwrite non-symlinks and
  fails on a missing target. --check reports drift without writing.
• .github/workflows/docs-symlinks.yml — runs --check plus a
  dangling-symlink check. Both were confirmed to fail on a deliberately
  mistyped symlink.

Regenerating drops 18 image symlinks no page references: eight hotSphere
figures linked twice over, one fillingElasticContainer figure, and
viscoTube-radialDisplacement.png, whose only reference upstream is malformed
(fixed by solids4foam/solids4foam#326 — hence checklist step 5).

Verified: the script reproduces the existing tree exactly apart from those
removals; the site builds under --safe with 414 pages; the internal
linkcheck reports no dead links; shellcheck is clean.

Why this PR reports no CI checks

Not a workflow bug. The PR is CONFLICTING on imported/solids4foam, so
GitHub cannot create the merge ref and therefore never generates
pull_request workflow runs. Checklist step 3 resolves the conflict, and the
checks will appear on their own. Until then both workflows have to be run by
manual dispatch.

Still outstanding

submodules: true on actions/checkout in markdownlint.yml — the last
Phase 0 item — is deliberately not in this PR. Turning it on surfaces 100
MD060 errors in the submodule's markdown, which solids4foam/solids4foam#326
fixes. It can go in once that has merged and the submodule pointer here has
caught up.

[philipcardiff]

Reminder before merging: this PR's submodule pointer is 7410d74, which
only exists on the docs-phase1 branch of solids4foam/solids4foam#324.

Merging as-is would point master at a commit that is not on development.
Merge #324 first, then update the pointer — the checklist in the description
has the exact commands. Marked as a draft until that is done.

[philipcardiff]

Correction to the checklist above: update-submodule.yml in the toolbox repo pushes a submodule bump directly to master here on every push to development. So step 2 is not a manual pointer update — once #324 merges, master picks up the right commit automatically, and this branch then needs master merged in with imported/solids4foam resolved in its favour. The description has been updated.

[philipcardiff]

Phase 2 folded into this branch (c1189e2, fdc6793): the manifest entries and symlinks for the twelve remaining solid model pages, plus a sidebar fix.

New hard ordering constraint. c1189e2 changes site_pages.liquid from sort_natural to the numeric sort filter, because the old string comparison puts 10 before 2 — already visible live, where the linear elasticity tutorials list as 1, 10, 14, 2, 3, 4, 5, 7. The numeric filter refuses to compare mixed types, so it breaks the entire build (comparison of Array with Array failed, not a per-page failure) against any submodule commit still containing a sort: xx placeholder. Four tutorial READMEs had them; they are fixed in solids4foam/solids4foam#324.

So this PR must not merge until the submodule pointer here includes that fix. That is already checklist steps 1-3, but the consequence of getting it wrong is now a hard failure rather than a cosmetic one.

Verified with the submodule temporarily checked out at the tip of #324: 130 pages built under --safe (118 before), solid models sidebar numbered 1-15 in order with no strays or duplicates, sync-docs.sh --check clean, no dangling symlinks, zero dead internal links, and markdownlint-cli2 clean over documentation/**.