Documentation improvements

[janheinrichmerker]

With this PR, I would like to combine our efforts to improve the package documentation for all three languages.
The Javadoc is already build as part of the Maven release. At this PR's initial state, I have added support for generating a Read the Docs-compatible Python documentation based on docstrings. Maybe we can also deploy the C documentation somewhere?

• Document Python code with docstrings
• Document Java code with Javadoc comments
• Document C code with Doxygen comments (this is at least partially done already, thanks to @TheMrSheldon)
• Deploy Python documentation
• Deploy Java documentation
• Deploy C documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (ef67be8) to head (7683899).

Additional details and impacted files

@@              Coverage Diff              @@
##             master      #35       +/-   ##
=============================================
+ Coverage     81.92%   93.27%   +11.34%     
=============================================
  Files             7        6        -1     
  Lines          1433      803      -630     
  Branches         56        0       -56     
=============================================
- Hits           1174      749      -425     
+ Misses          215       54      -161     
+ Partials         44        0       -44     

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[TheMrSheldon]

I will probably not be able to work on this for over a week. But in case you want to add a job for the C API, you only need to install doxygen, cmake, and graphviz and then configure CMake with TIREX_TRACKER_ONLY_DOCS=ON and TIREX_TRACKER_BUILD_DOCS=ON and let it build the tirex_tracker_docs target.

Otherwise I can do it when I get back and if you want this merged before that, you can remove C documentation from this PR and I create a separate one :)