docs: add mdBook docs-site, publish workflow, and release-notes tooling

Add a docs-site (mdBook) with a Gitea publish-docs workflow, a
release-notes generator script (release:notes) that turns a commit
range's TRACES into grouped notes, the background-audio feature spec,
and CLAUDE.md. Ignore docs-site build artifacts.

docs-site/book.toml

@@ -0,0 +1,25 @@
+# mdBook config for the published JellyTau documentation site.
+# The book's `src` is the repo `docs/` directory (see [build] below); this file
+# and SUMMARY.md live in docs-site/ to avoid cluttering docs/. The publish-docs
+# CI job copies SUMMARY.md into docs/ at build time, renders, and pushes the
+# result (plus the rustdoc API under /api/) to the orphan `gitea-pages` branch.
+[book]
+title = "JellyTau Documentation"
+description = "Requirements, traceability, and architecture for the JellyTau Jellyfin client."
+authors = ["Duncan Tourolle"]
+language = "en"
+# Sources live in the repo docs/ dir (one level up from this book root).
+src = "../docs"
+
+[output.html]
+default-theme = "navy"
+preferred-dark-theme = "navy"
+git-repository-url = "https://gitea.tourolle.paris/dtourolle/jellytau"
+edit-url-template = "https://gitea.tourolle.paris/dtourolle/jellytau/_edit/master/docs/{path}"
+
+[output.html.fold]
+enable = true
+level = 1
+
+[output.html.search]
+enable = true