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.

hosts/Darwin/README.md

@@ -1,51 +0,0 @@
-# macOS (nix-darwin) — `lyrathorpe-mac`
-
-Flake host: `lyrathorpe-mac` (`aarch64-darwin`). Apple Silicon Mac managed by
-**nix-darwin** from this same flake. Built via `mkDarwinHost` (single-user —
-macOS owns the account; identity still comes from the registry). Files:
-`configuration.nix`.
-
-## What this host is
-
-A macOS workstation. The interactive user environment (shell, git, editor,
-Claude) is the **shared `../../home` bundle** — the same modules the Linux hosts
-use — so the terminal experience matches. The Linux-only `desktop.nix`/`sway.nix`
-are intentionally left out. This host config covers the macOS-specific layer:
-system packages, Homebrew, and macOS UI defaults.
-
-## Package sourcing
-
-- **nixpkgs** (`environment.systemPackages`) for CLI tooling and libraries.
-- **Homebrew**, owned declaratively by `nix-homebrew` (Rosetta enabled for
-  x86_64 formulae). The `brews`/`casks` lists are **authoritative**:
-  `onActivation.cleanup = "zap"` uninstalls anything not declared. GUI apps are
-  casks (nixpkgs darwin GUI support is unreliable); a few version-pinned
-  toolchains and the PWA host stay on brew for continuity.
-- **Mac App Store** apps are **not** declarative: nix-darwin 26.05 runs
-  activation as root, and `mas` cannot reach the App Store session from root.
-  Install them by hand with `mas install <id>` from a GUI Terminal (the `mas`
-  CLI is in `environment.systemPackages`).
-
-## macOS integration
-
-- `security.pam.services.sudo_local` — **Touch ID for sudo** (and
-  `darwin-rebuild`'s sudo prompt), kept in `sudo_local` so it survives OS
-  updates. `reattach` pulls in `pam_reattach` so Touch ID works inside tmux
-  (which the terminals auto-start).
-- `system.defaults` — declarative dock / finder / global / trackpad preferences,
-  applied on activation and reversible. This is the main reason to run nix-darwin
-  beyond package management.
-- The JetBrainsMono Nerd Font is installed to `/Library/Fonts`; set it in
-  iTerm2 (Settings → Profiles → Text → Font) so the tmux statusline glyphs
-  render.
-
-## stateVersion
-
-`system.stateVersion = 5` (the nix-darwin state version, an integer — not a
-NixOS release string). Read `darwin-rebuild changelog` before changing it.
-
-## Apply
-
-```sh
-darwin-rebuild switch --flake .#lyrathorpe-mac
-```