Add readthedocs configuration

[parona-source]

• Configuration file is mandatory after readthedocs 10.8.0.

• [x] Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior, you should probably add or change a test case file under tests/test_cases/ and add it to the list under tests/test_definitions.py.
  If you've added or changed a command line option, you should probably add or change a test in tests/test_integration.py.

• [x] Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

[parona-source]

Fixes CI in https://github.com/PyCQA/pydocstyle/pull/656

[parona-source]

Tried to match the configuration in .github/workflows, but readthedocs doesn't support ubuntu-latest as an option so I had to pick a specific ubuntu release.

https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os