Migrate Openness Score documentation from Sphinx to mkdocs #451
Description
Currently the documentation is generated from docstrings to Sphinx. mkdocs have equivalent extension.
https://mkdocs.readthedocs.io/en/restructure-compat/user-guide/styling-your-docs/
Sticking with mkdocs, we only need to use one documentation framework, we can still use markdown, and we can reuse the know-how, pre-commit hooks (which are missing in this repo, BTW), etc.
https://mkdocstrings.github.io/ plugin can put Python docstrings into mkdocs as well
Such documentation should be hosted somewhere, e.g. GitHub pages.