Docs: Switch theme to enable page navigation and search

[MattHag]

The documentation on GitHub Pages lacks an overview and navigation structure to find all pages. Furthermore, it is common to have a search.

Solution

Switch to a common theme with left sidebar for navigation and built-in search like just-the-docs. This theme even supports mermaid for the diagrams.

With that change, the jekyll folder should be unified and merged into docs.

[MattHag]

@pfps That's something for you, as the doc creation is not public

[MattHag]

#2516 is an example, that's solveable with enhanced and easy to navigate documentation.

[MattHag]

MKDocs seems ro be better, when adding developer docs as well
https://github.com/squidfunk/mkdocs-material/tree/master

This seems more in line with #2503.