Documentation using Sphinx+Exhale+Doxygen (revised)

[msmk0]

This moves the documentation build to use Sphinx with the Breathe/Exhale extensions. Doxygen is called automatically by Sphinx and is used only to extract the existing source code documentation.

• The documentation now integrates auto-generated API documentation and manually written texts in a single website with the ability to interlink.
• Most of the documentation that is currently on the website is imported back into the docs directory in this repository.
• The README.md is streamlined and contains mostly usage and code organization information and provides links to other sources to avoid duplication.

Please note:

• The local documentation build, e.g. via make docs, uses a fixed path in-source build (in contrast to the configurable out-of-source build directory for the code) to be compatible with https://readthedocs.org. You might need to remove docs/{_build,api} manually for a clean build.
• The Sphinx build with API generation takes O(10min). For faster iteration a make docs-without-doxygen target is available that only builds the manually written parts. A make run-doxygen target is available to run doxygen standalone to simplify debugging the Doxygen config.

This is an updated versions of https://gitlab.cern.ch/acts/acts-core/-/merge_requests/614.

Closes #44 and supersedes/closes #96.

[msmk0]

@paulgessinger I still need to do a few content fixes but you can already start to have a look.

[codecov]

Codecov Report

Merging #197 into master will increase coverage by 0.44%.
The diff coverage is 58.22%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   44.22%   44.66%   +0.44%     
==========================================
  Files         365      369       +4     
  Lines       18239    18454     +215     
  Branches     8691     8782      +91     
==========================================
+ Hits         8066     8243     +177     
+ Misses       4928     4874      -54     
- Partials     5245     5337      +92     

Impacted Files	Coverage Δ
...clude/Acts/Vertexing/AdaptiveMultiVertexFinder.hpp	78.57% <ø> (ø)
...include/Acts/Vertexing/FullBilloirVertexFitter.hpp	75.00% <ø> (ø)
...e/include/Acts/Vertexing/IterativeVertexFinder.hpp	76.92% <ø> (ø)
...e/include/Acts/Vertexing/IterativeVertexFinder.ipp	20.86% <0.00%> (ø)
...nclude/Acts/Vertexing/TrackDensityVertexFinder.hpp	75.00% <ø> (ø)
...nclude/Acts/Vertexing/TrackDensityVertexFinder.ipp	100.00% <ø> (ø)
Core/include/Acts/Vertexing/ZScanVertexFinder.hpp	81.81% <ø> (ø)
Core/include/Acts/Vertexing/ZScanVertexFinder.ipp	64.70% <ø> (ø)
...clude/Acts/Vertexing/AdaptiveMultiVertexFinder.ipp	44.59% <42.85%> (+11.49%)	⬆️
...include/Acts/Vertexing/GridDensityVertexFinder.ipp	45.45% <45.45%> (ø)
... and 10 more

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[msmk0]

@paulgessinger @HadrienG2 @asalzburger This is now ready to go in. I updated the content a bit and streamlined the README.md to remove duplicate/obsolete information. The PR description has also been updated to reflect the final set of changes.

[paulgessinger]

Codecov seems not to post the results, and I'm not sure what's wrong with the review check. I'm overriding this.