feat(docs): self-maintaining llms.txt generated from nav.yml

[aheritier]

What

Adds a spec-compliant /llms.txt to the edge documentation site (docker.github.io/docker-agent), generated at Hugo build time from docs/data/nav.yml. Adding a page to the nav automatically adds it to llms.txt — it self-maintains, deployed by the existing docs-deploy workflow with no new CI or cron.

Why

The docs describe an agent runtime, so their audience is disproportionately LLMs pulling them into a coding context. llms.txt gives models a curated, single-file index of the documentation. docs/data/nav.yml is already the hand-curated canonical index, so driving generation from it keeps the two in lockstep with zero extra maintenance.

Scope note: This covers the edge/preview site only (which tracks main). The stable docs on docs.docker.com already publish their own llms.txt, llms-full.txt, and per-page .md companions. Links here intentionally point at the github.io edge URLs. A follow-up may propose enumerating docker-agent sub-pages in docker/docs's own llms.txt.

Changes

File	Change
docs/hugo.yaml	Register a plain-text llms output format, attached to the home page via outputs.home (without which it is never rendered). disableKinds untouched.
docs/layouts/home.llms.txt	Template: H1 + blockquote summary + one ## section per nav group with - [title](url): note links (absolute via absURL, note from each page's front-matter description). Flattens both items and groups[].items; hard-fails the build if a nav page is unresolvable or its description is empty.
docs/layouts/_partials/footer.html	llms.txt discoverability link.
scripts/docs-check-llms-txt.sh	CI-invoked semantic validator: rebuilds the site and asserts llms.txt matches nav.yml's sections, titles, order, count and per-entry URL (position-for-position). Catches template regressions a green build alone would miss.
.github/workflows/docs-lint.yml	New llms-txt-check job (pinned Hugo 0.163.0, matching docs-deploy/docs-a11y).
docs/STYLE.md	Documents generation, the description requirement, and the CI check.

Validation

• Local + CI-pinned Hugo 0.163.0 build → llms.txt OK: 102 entries across 7 sections, matching docs/data/nav.yml
• Validator verified to catch: dropped entries (broken groups[].items traversal), duplicated/wrong same-origin URLs, missing/whitespace-only descriptions, and section-order drift
• shellcheck clean; ran 25× consecutively with no flakiness
• Blast radius: low — purely additive output format; existing HTML output, nav, and deploy pipeline untouched. Rollback is a plain revert.

🤖 Generated with human review via the multi-agent workflow (implement → review loop → merge-ready).