Flatten rbtools docs a bit.

The structure of the rbtools docs made it really unclear how to actually see
information about using the tools, since the table of contents just had stuff
about installation.

I've pulled the rbt/index and api/index files into index itself, under a "Using
RBTools" section. At least this way, the top-level TOC has something other than
"Installation", and there are a bunch of links visible as soon as you see the
page to the specific commands, instead of having to drill down through several
pages.

While I was doing this, I pulled in the info from "Automating post-review" in
the old tools section of the Review Board user manual, which seemed to be the
only thing present in that that wasn't here.

Testing done: Built html docs and looked through them.

Reviewed at https://reviews.reviewboard.org/r/4997/