fix: fix docs warnings in Sphinx 3.0+

[plamut]

Fixes #69.

This PR fixes the docs generation warnings that occur in the latest version of Sphinx, causing the builds to fail (we treat warnings as errors).

To test the PR, generate the docs with and without the changes here. With the changes, the docs build should not fail anymore, while the docs themselves are not changed (apart from the changelog that had some structural issues).

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)