Create new reference documentation layout

[ischeider]

Description and Context

This PR introduces the new layout for the reference sections. The basic layout has been provided by @gilrrei (thanks!), now they are integrated into the documentation. The new style is very informative which unfortunately means that the reference documentation becomes very large and complex. Hence, the built-up of the page was very slow, and I split the reference documentation in different chapters.

As of now we have:

• Cell types (was already available previously)
• Spatial discretization
• Material information
• Boundary conditions
• general parameters

The differentiation is done by simple string search, which is done in the routine create_markdown_documentation within https://github.com/ischeider/4C/blob/17ef391e65c36254bf9ed350a1bdf68bb1c23430/utilities/four_c_python/src/four_c_documentation/make_documentation.py . Particularly, please check whether we should use a different mechanism or split the chapters differently.

In addition, this PR removes large parts of create_rtd. Only the resources for the cell type definitions are still contained in this tool. If anybody knows how to get the information about node, line and face numbering of the different elements in a different way, we can dump the whole tool (but deleting this information is not the solution IMO).

[ischeider]

One additional comment:

A particular feature introduced here is the listing of all test files that contain a particular section (which by default is collapsed) - thanks again, @gilrrei .
However, some section are used so often that I think a listing is not useful (e.g., SOLVER 1 or MATERIALS). Therefore, in make_documentation.py I introduced a constant TOO_MANY_TESTS_TO_SHOW, which is set to 200 by now. Any discussion whether this is meaningful is welcome.

[sebproell]

Very cool! I downloaded the new docs and had a look. I believe this PR is in good shape to be merged as this is much better than what we had before. We can always improve the styling or behavior of certain sections, but it is good to get the new infrastructure in place. Many thanks @ischeider @gilrrei!

[gilrrei]

Thanks for making my script mess usable 😅 I think this is already a great improvement 🎆

I think for some reason the FUNCT section is missing 🤔 (edit: my bad)

[ischeider]

This commit contains all the reviewer comments (thanks @gilrrei @sebproell !) except a little formatting and the FUNCT stuff (don't know yet how to include this less hacky)

[ischeider]

I also added the description for the reference chapters. Please have a look.

[ischeider]

Resolved the merge conflict, and added the comment about MATERIALS by @sebproell above.

[gilrrei]

A diff of 900 lines, so nice