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>

docs/book.toml

@@ -0,0 +1,12 @@
+[book]
+title = "yt-offline"
+description = "Self-hosted archive for YouTube, TikTok, Twitch and more. Setup, usage, anti-bot, troubleshooting, and architecture."
+authors = ["InannaBeloved"]
+language = "en"
+src = "src"
+
+[output.html]
+default-theme = "navy"
+preferred-dark-theme = "navy"
+git-repository-url = "https://codeberg.org/anassaeneroi/yt-offline"
+edit-url-template = "https://codeberg.org/anassaeneroi/yt-offline/_edit/main/docs/{path}"