The Tinted Theming repository is now packaged in Nixpkgs, so we
don't need to list other ways to obtain it.
Also simplified the links to the preview as `$XDG_CONFIG_HOME`
is rarely modified.
This was more of a developer tool than part of the documentation, so it
makes sense to keep it off the main site. Also when modifying the
palette generator, the documentation on the site would become out of
sync with local changes anyway.
