Add Doxygen-style comments to RRGraphView and RRGraphBuilder, and make new VPR API tab

[vaughnbetz]

We have nice new rr-graph APIs, so we should auto-generate API documentation on them.

Proposed Behaviour

New RRGraphView and RRGraphBuilder tabs on the vtr api tab in read the docs. Also should add some high-level comments (if they aren't there already) explaining exactly when each of these should be used.

Current Behaviour

Good comments, but I don't think they're doxygen-compatible and we don't build docs on them.

Possible Solution

Reformat comments to be doxygen-compatible, and add to the docs page.

Context

The comments and use of these classes will be more understandable if they're on doxygen.

[vaughnbetz]

Cleaning up my old issues! @tangxifan : This all looks good, but it would be good if the RRGraphView class had a high level comment on what it is used for that showed up in the documentation. RRGraphBuilder and RRGraphSpatialLookup already have those high level comments.

[tangxifan]

@vaughnbetz No problem. I will work on this.