Add user guide/examples/tutorials

[mtreinish]

What is the expected enhancement?

Right now our documentation is mainly focused on api references with some small examples in the class documentation and release notes. It would be great if we had a user guide in the documentation top explain at a higher level how to interact with retworkx, working with graphs/networks, and accomplishing some concrete tasks using retworkx. Some good examples of documentation like this:

• https://qutip.org/docs/latest/guide/guide.html
• https://numpy.org/doc/stable/user/absolute_beginners.html

Writing documentation like this will also be good for exposing any API or functionality gaps in retworkx.

[IvanIsCoding]

I am working on this but feel free to join this issue, contributions with guides and tutorials are always welcome

[mtreinish]

I got a start to this in https://github.com/Qiskit/retworkx/pull/537 I think there is still a lot of room for expansion after #537 though so I'm not marking this as closed by that PR. I do think that gives us a model and a place for adding more guides and tutorials like that to the documentation though. So we can build from that as a starting point and close this when we think we have enough here.