Move docs into a subdirectory.

Move the rbtools documentation into a subdirectory under the docs
directory. This was done to allow for other sphinx docs configurations
such as releasenotes.
Assign documentation copyright to Beanbag, Inc.

List Beanbag as the copyright holder and fix a style error in the
sphinx configuration.
Port rbtools release notes from Review Board repo

Introduces a releasenotes sphinx configuration, and includes
all previous release notes from the Review Board tree.