Use Sphinx for documentation instead of mkdocs

[kretep]

Migrate documentation from mkdocs to sphinx. Creating a separate PR for this, to make the one with mixed improvements (#79) more manageable.

Biggest advantage of moving to sphinx is that the API reference can be automatically generated (this didn't work with mkdocs).

[kretep]

The "build" action is failing as expected; it is replaced by "build-and-test" in #79 .

[vedran-kasalica]

The "build" action is failing as expected; it is replaced by "build-and-test" in #79 .

To fix the issue, should we then update or remove .github/workflows/python-package.yml?

[vedran-kasalica]

I agree with the idea of switching to Sphinx, and the changes in the PR reflect that well.

[kretep]

To fix the issue, should we then update or remove .github/workflows/python-package.yml?

We can remove it

[vedran-kasalica]

To fix the issue, should we then update or remove .github/workflows/python-package.yml?

We can remove it

Do you prefer doing it before or after we merge this PR?

[kretep]

Its removal is also part of #79, so I was thinking to just not do it twice, although there won't be any conflicts if we remove it here