Documentation

[arcnmx]

• [ ] gir can generate docs apparently rustdoc-stripper however seems onerous to use... see gir-docs branch for a prior attempt?
• [x] CI must maintain multiple branches worth of docs: one for each major branch, one for the latest crates.io release just use the toml version as a prefix and use symlinks/redirects?
• [x] set up html_root_url per branch, add reminder to Cargo.toml
• [x] examples/README missing from generation?
• [ ] remove glib-signal crate from docs generation (rustdoc leaves old crate roots as-is, it's ancient and unused)