docs: add readthedocs doxygen wrapper

[smlng]

addressed #118

This may need some work. Currently it shows (lots of) errors during generation, and the output is single-page -> hence, a bit unstructured.

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 3.54%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage    67.7%   64.15%   -3.55%     
==========================================
  Files          16       14       -2     
  Lines        2149     1928     -221     
  Branches      369      367       -2     
==========================================
- Hits         1455     1237     -218     
+ Misses        694      691       -3

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 621aee0...7e2f589. Read the comment docs.

[waehlisch]

@smlng thanks. Can you provide a readthedocs link to a preview? When do you will fix the errors?

Btw, now we have two Doxyfile, which might be confusing. Should be cleaned up later.

[smlng]

Regarding the 2 Doxyfiles: I'm not sure if they can be merged into one, the problem is that doxygen/Doxyfile is used by the Cmake toolchain and docs/Doxyfile is for the RTD intgeration, which needs some adaptions for output params and paths.

[smlng]

@waehlisch I put a demo side online, have a look here.

Regarding the errors, I see during doc generation: most of them are related to undocumented code parts (functions, parameters, and so on), some are readthedocs + breathe related, but not severe.