docs: move prose documentation into docs/ so the docs site publishes it

The docs-site build syncs this repo's README.md and docs/ into the site
tree; nothing else is copied. All prose apart from the README therefore lived
outside the sync and never appeared on https://docs.lyrapup.pet/nixfiles/, and
the one page that did publish carried 18 link targets that resolved to nothing.

Moves:

  home/README.md           -> docs/shell.md
  home/KEYBINDINGS.md      -> docs/keybindings.md
  hosts/<Name>/README.md   -> docs/hosts/<name>.md

docs/.pages and docs/hosts/.pages give the awesome-pages plugin an explicit
order; new pages are picked up by the trailing '...' without an edit.

Links are rewritten so a single URL is correct in both Gitea and the published
site: absolute Gitea source URLs for .nix files and directories, relative links
between pages under docs/, and absolute docs.lyrapup.pet URLs from the root
README, which the build republishes at a different depth from the rest of the
tree. In-code comments that pointed at a moved README are updated to the new
path.

The README gains a Documentation section covering the sync contract and the
linking rules, and CLAUDE.md carries the short version so future edits do not
reintroduce unsynced pages or dead links.

Verified by reproducing the docs-site assembly locally against its pinned
toolchain (mkdocs 1.6.1, mkdocs-material 9.7.7, awesome-pages 2.10.1): pages
render at the URLs used above and in the declared order.

docs/hosts/.pages

@@ -0,0 +1 @@
+title: Hosts