add basic sphinx & readthedocs infra

[jakkdl]

Let's see if this works~

[jakkdl]

current RTD fail is missing dependencies to be able to import flake8_async, so need to install those, or scraping the __version__ line manually if we're not gonna need to access the package for other reasons. And then copy over a bunch of stuff from the readme and actually write/LLM-generate some docs

[jakkdl]

Nice. Now I just gotta get some docs

[jakkdl]

Slowly closing in on this being an improvement/on-par to status quo.

[jakkdl]

Okay I think this is worth reviewing now. You can view the docs at https://flake8-async--223.org.readthedocs.build/en/223/ (or click through from the CI RTD check) Next step would be breaking out rules-specific pages with a la ruff, but I'd need to spend some solid time figuring out a decent way of doing that which doesn't involve me manually creating a million different .rst files. I've been digging through the source for the rust docs to try and see how they do it but not been able to so far.

Other minor TODOs:

• Move CONTRIBUTING.md to the docs
• Get the docs linked in the readme, on PyPI, etc
• Strip down README.md
• Push a release

But I think we can put those in a separate PR. Would be nice to get this merged so I can update docs/ when writing feature PRs.