Add distro to documentation

[clalancette]

Branch off the documentation for this release, and add it into the configuration. Something like:

• https://github.com/ros2/ros2_documentation/pull/2424
• https://github.com/ros2/ros2_documentation/pull/2435

Also, edit the front page of https://docs.ros.org/ to point to documentation. Basically ssh rosbot@docs.ros.org , cd /var/www/docs.ros.org , edit index.html to add new distribution

[clalancette]

This is so early in the process so that we can do some testing with YATM for generating test cases, which requires the documentation to be available.