The release notes used to be an appendix in the manual. After
converting to markdown that appendix got lost. This commit
reintroduces the release notes into the manual.
The `nixos-render-docs` tool outputs XHTML formatted content. In
order to convince browsers like `firefox` to treat the data as XHTML
the extension must be `.xhtml` and not `.html`.
Using the XHTML-formatted content as HTML is mainly an issue with
self-closing `<a />` tags.
