Update documentation

PyonFX documentation is starting to be a bit dated: inconsistent docstrings, poor explanations and lack of examples are an obstacle for new users. Plan: * For docstrings style/consistency: https://github.com/astral-sh/ruff; * Use Mkdoc (and Material for MkDoc as a theme) to generate the Static Site, instead of ReadTheDocs (which should be updated to point to GitHub); * Follow the Diátaxis documentation framework to organize docs into four main sections: Tutorials, Guides, Reference, Explanation; * The pages should have images/videos to showcase the features; * Add build & deploy through GitHub Actions.