Redesign of html pages and repo structure

[domna]

For proper navigation it would be useful to have the html pages more approachable and selectable. Therefore, the html page generation should be redesigned.

Features

• Make selection for requiredness, so that one can only look at required, recommended, optional fields
• Selection to display the fields of a base class on the page it is used (maybe even with required, recommended, optional switch as soon as we have base class inheritance (BCI))
• Make our html representations a separate repo, so we don't have to maintain a separate styling than NIAC in the same repo (makes interaction with NIAC a lot easier!)
• Better structuring: This might be a bit offloaded to the pynxtools docs as a general point of first interaction but I think it would also be helpful to have more guidance when one comes first to the documentation.

Maybes

• Use mkdocs instead of sphinx? (Problem: nxdl docstrings are written in rst!)

@FAIRmat-NFDI/areab Feel free to comment and extend