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/introduction.md

@@ -0,0 +1,40 @@
+# yt-offline
+
+A self-hosted media archive for YouTube and friends. Paste any URL, it
+routes the download to the right folder by source, tracks what you've
+watched, and plays everything back from a desktop GUI **or** a browser —
+even offline, or after the source video is taken down.
+
+Built on [yt-dlp](https://github.com/yt-dlp/yt-dlp); written in Rust as a
+single binary that is **both** a desktop app and a headless web server.
+
+## What it backs up
+
+| Platform | Channels | Playlists | Single videos |
+|---|---|---|---|
+| YouTube | ✅ | ✅ | ✅ |
+| TikTok | ✅ | — | ✅ |
+| Twitch (VODs + clips) | ✅ | — | ✅ |
+| Vimeo | ✅ | ✅ | ✅ |
+| Bandcamp | ✅ (artist) | ✅ (albums) | ✅ (tracks) |
+| SoundCloud | ✅ | ✅ (sets) | ✅ |
+| Odysee | ✅ | — | ✅ |
+| Anything else yt-dlp accepts | `other/` | `other/` | `other/` |
+
+## Why it exists
+
+Tartube is the mature open-source yt-dlp GUI and the benchmark in this
+space. yt-offline matches its feature set while adding things Tartube
+doesn't have: a real web UI reachable from any device, a single-binary
+distribution with a bundled toolchain, a modern security model
+(password-gated UI, Argon2, rate-limited login), a built-in anti-bot
+stack (TLS impersonation + Proof-of-Origin tokens), and one-click
+post-download format conversion.
+
+## How to read these docs
+
+- New here? **[Installation](./installation.md)** →
+  **[First run](./first-run.md)** → **[Downloading](./downloading.md)**.
+- Hitting captchas or "Video unavailable"? Go straight to
+  **[Anti-bot](./anti-bot.md)** and **[Troubleshooting](./troubleshooting.md)**.
+- Hacking on it? **[Architecture](./architecture.md)**.