Generate C/Fortran API docs with Doxygen

[sloede]

To generate the docs, go to docs/doxygen and run doxygen without any arguments. You can then view the result by opening the file docs/doxygen/build/html/index.html in your browswer.

This is just a very early draft after playing around some (too much) time with Doxygen 🙄 One particular challenge is the fact that we have to document APIs in two languages (actually three, if you include Julia), and that some function names are identical. I believe that this similarity is the reason why, e.g., the documentation for the Fortran interface might not show up properly (or maybe it just doesn't support Fortran interfaces).

Instead of using Documenter.jl for the main docs and Doxygen for the C/Fortran API, we could also consider to use Doxygen only for the C part and using something like FORD for the Fortran part.

Anyhow, this is as far as I'm going to take it with docs. @bgeihe feel free to adopt this PR, add to it, or just close it again if you don't like it - I just didn't want to get the time I put into this go to waste entirely :-)

The following things one would have to do if willing to make this PR work:

• Figure out how to properly document the Fortran API
• Auto-generate the docs as part of the call to Documenter.jl and put the result in a subdirectory
• Link to C/Fortran API from main docs

[codecov]

Codecov Report

Merging #52 (4cd16ff) into main (5ba6bf2) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main      #52   +/-   ##
=======================================
  Coverage   70.12%   70.12%           
=======================================
  Files           5        5           
  Lines         164      164           
=======================================
  Hits          115      115           
  Misses         49       49           

Flag	Coverage Δ
unittests	70.12% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/trixi.c	65.88% <0.00%> (ø)
src/trixi.f90	71.42% <ø> (ø)

[sloede]

Preview the docs here: https://trixi-framework.github.io/libtrixi/previews/PR52

Go to "Reference" ➡️ "C/Fortran", where you will find a link to the doxygen-generated docs: https://trixi-framework.github.io/libtrixi/c-fortran-api

EDIT: The direct link to the C/Fortran API docs will not work anymore since we use proper subdirectories now.

[benegee]

I would like to check the output of moxygen, but I can do this in a separate PR as well.

[sloede]

I would like to check the output of moxygen, but I can do this in a separate PR as well.

The current state of the PR is that everything seems to work as it should, including the placement of the C/Fortran API into the proper folder such that it respects the versioning of Documenter.jl.

I thus recommend we go ahead and merge this PR in its current state (after a review by you, of course). All improvements can then be iterated on in a new PR.