Create a colcon extension for building documentation

[clalancette]

It would be nice if users could just use colcon to build the documentation for all of the packages in their workspace. This will likely require a new colcon verb (colcon doc), along with the appropriate backend to call out to rosdoc2 to generate the documentation.

[mikepurvis]

We have the bones of a colcon-document verb (basically just a port of https://github.com/mikepurvis/catkin_tools_document) though currently it just invokes doxygen and sphinx directly, eg it's the ROS 1 rosdoc behaviour rather than interacting with rosdoc2. If there's interest from the community in collaborating, we can likely share our work-in-progress.

FYI @guillaumeautran