Create sphinx-based website

[tkittel]

We have discussed this for years, but apparently there is not a dedicated issue for it yet: NCrystal needs a proper website. It should be based on sphinx and live in the main repo similarly to how it is done for e.g. the simplebuild project: https://mctools.github.io/simplebuild/

There are many many tasks to mention in this regard, but I am opening this issue to have a place to note them down.

[tkittel]

Related to the data library page which @MilanKlausz is working on in #168.

[tkittel]

Idea: show various charts on development subpages with: https://pypi.org/project/sphinxcontrib-mermaid/ (e.g. dependency graph between the coding modules that will be introduced in v4.0.0).

[tkittel]

We also need the tutorial notebooks integrated. And a "material cookbook" page, and cfg-string docs, and CLI tools argparse docs, and discussion about the physics, etc. etc.