Add documentation for the rbt commands

A page for each command has been added listing a short description and
the options which the command accepts. Eventually these should be
expanded to provide more in depth documentation.

Also, the post-review documentation has been ported from the Review
Board code base. This also served as the starting point for the rbt
post command's documentation.

Additionally, a couple of unused options, which were noticed while
documenting, were removed from the post command.

This is dependent upon /r/3296

Built the documentation using 'make html' and visually inspected.