Migrate documentation to MyST

[schmoelder]

This PR is part of a larger reorganization and unification of the documentation (see #4) and migrates the documentation from Sphinx / RST files to Myst, a modern Markdown based tool for technical and scientific publication.

As discussed in the Focus Meeting, Myst does not currently support multiple versions out of the box, which is problematic given the ongoing breaking changes. The temporary solution is to migrate to ReadTheDocs, which supports this feature, and redirect from cadet.github.io.

To-dos

• Formatting
• Bibliography

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

[jbreue16]

The best temporary solution to the multiversion probelm that would allow merging this quickly is in my opinion: keep the current GitHub.io website as is and only add a redirect to readTheDocs for the master version, i.e. add the redirect to the banner where it says "master version" and to the master branch on the sidebar. That should be possible to implement, what do you think?