Emit description and keyword metadata from rustdoc

This teach rustdoc to add `<meta name="description">` and `<meta name="keywords">` tags to crate docs. Description is important for search engines because they display it as the page description. Keywords are less useful but still generally recommended.

This also changes the "stability dashboard" link to just say "stability", because the current link takes up a lot of space.

cc https://github.com/rust-lang/rust/issues/12466