Add initial Sphinx docs

[ConorMacBride]

This adds an initial version of the pytest-mpl Sphinx docs. There's still a lot of things to change, but might be good to get an initial latest version published. It uses matplotlib sphinx theme. The content all mostly copied and pasted from the current readme.

I've added a custom sphinx plugin for generating the HTML summaries of some of our tests and including them in the documentation similar to https://macbride.me/pytest-mpl/. Also added a custom plugin to make some fixes to the sidebar, although we should probably upstream that to the matplotlib sphinx theme eventually, as it will be useful to other non-native doc sites.

MetPy has some amazing infrastructure for publishing versioned Sphinx docs. Hope you don't mind, I've included a simplified workflow based on that in this PR, although maybe we should include some of the more complicated version publishing automations, or just manually edit the gh-pages branch on every release? I currently have it set make the docs available at https://matplotlib.org/pytest-mpl/latest/ on every push to the main branch, with no special treatment of tags.

You can see what the docs currently look like here: https://macbride.me/pytest-mpl/latest/index.html

Closes #157

[Cadair]

This looks good. Beyond my general disdain for reimplementing RTD with gh-pages I think we can ship this?