Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
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.
