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.
(cherry picked from commit 6fc71dc563)
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.
