Add summary of the retworkx benchmarks to the documentation

[IvanIsCoding]

What is the expected enhancement?

Add a summary of the results from https://github.com/mtreinish/retworkx-comparison-benchmarks to the documentation. The ultimate goal is to have a page to refer users to if they ask about benchmarks.

The information present in the benchmarks can be useful to users, mainly when comparing retworkx with other libraries. The results can be static i.e. we do not expect the benchmarks to run on CI or even to be run for every release.