add plotting cookbook to docs

[atravitz]

Unsure if this should be a tutorial or cookbook, but having something in our docs to give users a starting point would be valuable.

[nffaruk]

Maybe this should be a separate issue, but there should be functions easily accessible from the Python API that can reproduce the quickrun CLI plots that are autogenerated (e.g. convergence, overlap and transition matrices).

Edit: I see these functions are in openfe.analysis.plotting, but they aren't mentioned in the API Reference docs