Documentation site: mdBook user + contributor guide (2.2)

An mdBook under docs/ rendering eight pages: introduction,
installation, first-run/config, downloading, an anti-bot guide that
captures the hard-won cookies/curl_cffi/POT/player-client knowledge,
troubleshooting (the nine error classes + non-download issues),
architecture (the two-front-ends/one-engine design, settings flow,
layout invariant, persistence), and packaging.

- .forgejo/workflows/docs.yml builds the book and publishes docs/book/
  to the `pages` branch for Codeberg Pages on docs/ changes.
- docs/book/ is gitignored (rendered output).
- README: fix the stale backup.directory comment — every platform nests
  under the one library root, not a channels/ sibling split.
- Add CLAUDE.md (repo guidance for Claude Code).

Builds clean with mdbook 0.5; intra-doc anchor links verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -192,7 +192,7 @@ fields are also editable in Settings.
 
 ```toml
 [backup]
-directory      = "/path/to/library/channels"   # YouTube root; siblings hold other platforms
+directory      = "/path/to/library"            # library root; every platform nests under it (channels/, tiktok/, …)
 max_concurrent = 3                              # parallel yt-dlp processes
 use_bundled_ytdlp = false                       # true = use the venv set up by the Install button