Auto merge of #117888 - notriddle:notriddle/releases, r=Mark-Simulacrum

doc: add release notes to standalone doc bundle

Preview: http://notriddle.com/rustdoc-html-demo-5/release-notes/releases.html

This is a workaround for #101714 on top of being a useful addition in its own right. It is intended to change the "canonical URL" for viewing the release notes from GitHub, which is relatively slow, to a pre-rendered HTML file that loads from the same CDN as the standard library docs. It also means you get a copy of the release notes when installing the rust-docs with rustup.

Author: bors