Branch out for galactic

[fmauch]

With #339 and #285 it would make sense to branch out for galactic for now. Before we do this, however, I'd like to merge a couple of other things first to reduce the amount of backports necessary:

• Add resource files from ROS World. #226
• Use upstream fts_broadcaster #304
• Update README regarding MoveIt setup #333
• Add sphinx documentation #340

[fmauch]

Branch out happened, updating READMEs and workflows:

• Updated workflows for galactic branch #358
• Update README and workflows as galactic is now branched out #359