rustdoc: Use more descriptive description metadata.

This text appears in and improves search results.

cc #12466
rustdoc: Put field instantiation in declaration order.

cc #12466
rustdoc: Emit keywords for all crate pages

cc #12466
rustdoc: Just "stability" instead of "stability dashboard"

The words "stability dashboard" take up too much space on small screens.
auto merge of #16254 : brson/rust/rustdocmeta, r=aturon

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