Move the README's docs into docs/, and keep a short README that sends readers to drivine.org #226

The README has grown into the whole manual. drivine.org renders the docs far better: a page per topic, a sidebar, search, and the same layout for all three libraries. So the long content moves into this repository's docs/ folder, one file per section, and the README becomes a short landing page: what Drivine is, install, a short quick start, and links to drivine.org/py and docs/.

drivine.org's docs are generated from these files (drivine's website/scripts/import-readmes.mjs), so every page and URL stays as it is.

Done in #223, held until the release it overlaps is published.