Add some overview to the RRGraphView doxygen documentation

[vaughnbetz]

Proposed Behaviour

We should give a high level overview of the RRGraphView data structure / API in our doxygen documentation.

Current Behaviour

Currently we dive right into methods.

Possible Solution

Explain the RRGraph describes the programmable routing fabric of the FPGA (all wires and switches). RRGraphView provides a read-only interface, which tools like the router, graphics, and statistics need.
Should also explain that the fundamental data structures is a graph (nodes and out-going edges) with extra information like the location within the chip and electrical parameters added to help make algorithms more efficient, draw the chip, and estimate delays.

[vaughnbetz]

Re assigning to @tangxifan . This might be a duplicate though!

[vaughnbetz]

Is a duplicate of the remaining work in #1834 . We can close both together.

[tangxifan]

@vaughnbetz Indeed. We may close this one and keep #1834 open. My team will follow up. A PR will happen this summer.