Update gh-actions config

[jwdink]

Would be great to add a few standard enhancements to the setup:

• Cacheing: It would be good to cache certain steps. For example, currently we re-install pytorch from scratch (which is actually a fairly large download).
• Build Docs: Building the docs with gh-actions would be helpful because the 3gb memory of readthedocs is limiting for one of the example notebooks. In fact, I haven't been able to re-build the documentation for a long time now, because this example doesn't fit in RTD memory. (Related to https://github.com/strongio/torchcast/issues/3 as we would host these.)
  • Along the way, we could switch the theme to strong's theme.

Feedback welcome on anything else that is standard / best practice.