Update Sphinx config to V2, and enable docs builds on PRs

[connortann]

I saw by email that our ReadtheDocs config format is deprecated, and the build is currently failing. This change should fix it. Link: https://blog.readthedocs.com/migrate-configuration-v2/

Example build now passes successfully: https://readthedocs.org/projects/resqpy/builds/22562103/

I also enabled the setting to build docs on all PRs. This may not take effect for this PR, but should work for future ones.

https://docs.readthedocs.io/page/guides/autobuild-docs-for-pull-requests.html

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7143897) 81.40% compared to head (59e4153) 81.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #767 +/- ## ========================================== - Coverage 81.40% 81.40% -0.01% ========================================== Files 190 190 Lines 32811 32811 ========================================== - Hits 26711 26710 -1 - Misses 6100 6101 +1 ```

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