docs: add sphinx documentation and add missing documentation

[d4l3k]

This adds sphinx documentation following the setup from https://github.com/pytorch/torchx.

This will build it in Github Actions CI and then deploy it.

Notably, this only builds the latest version of the documentation and not historical versions.

Test plan:

cd docs
pip install -r requirements.txt
make livehtml
make html

CI

[d4l3k]

CI passing -- landing to main to test deploy