This updates the documentation to provide a simpler, more descriptive
front page, and to better organize some of the configuration and API
pages.

The front page starts off with a more fleshed-out description of
RBTools, with simple sections on installation and configuration. Each of
these point to the relevant pages for the topic.

The Installation section no longer has instructions for each OS, but
rather links to the new RBTools Downloads page, which has more modern
instructions and installers.

There's now two configuration pages: One for repository configuration,
and one for user configuration. The latter includes configuration of
aliases, which is no longer its own page.

All the directories now have their own indexes, which are properly
chained into toctrees, helping with the organization.

Some of the text in various sections was also cleaned up just a bit to
flesh out the content and make things more readable.

Built the docs and browsed around. Checked that links were still working

after the intersphinx updates.
The docs building didn't show any warnings.