Add Sphinx docs for `cuda.pathfinder`

[leofang]

btw @rwgk could you create an issue to track the need to add docs for pathfinder? We need a Sphinx-rendered docs.

Originally posted by @leofang in #723 (comment)

[leofang]

We need

1. https://nvidia.github.io/cuda-python/cuda-pathfinder/
2. Add cuda.pathfinder to the landing page https://nvidia.github.io/cuda-python/ and the root README.md