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/src/SUMMARY.md

@@ -0,0 +1,16 @@
+# Summary
+
+[Introduction](./introduction.md)
+
+# Using yt-offline
+
+- [Installation](./installation.md)
+- [First run & configuration](./first-run.md)
+- [Downloading](./downloading.md)
+- [Staying ahead of YouTube's bot detection](./anti-bot.md)
+- [Troubleshooting](./troubleshooting.md)
+
+# Contributing
+
+- [Architecture](./architecture.md)
+- [Packaging](./packaging.md)