Handle Sphinx context injection deprecation

[rebeccabilbro]

We received notification from Read the Docs that they are deprecating Sphinx context injection at build time. This PR updates the Sphinx configuration in the docs per the instructions provided by RTD. I've also updated the docs sphinx requirements to ensure the fix works as expected. After making these changes locally, I ran make html from the docs folder, and the docs seemed to build correctly. I'm not sure how else to test this than just opening a PR.