Correct me if I'm wrong, is YARD not the same as RDoc?
It's not based on RDoc but a total rewrite?
So if I'm using YARD, I don't have to bother with RDoc right?
In short: Yes. (To all three questions.)
Note that YARD is mostly backwards-compatible with RDoc, at least as far as the markup is concerned. (RDoc plugins OTOH don't work with YARD.) So, you can actually use YARD to process your legacy RDoc files. YARD is mostly a proper superset of RDoc.