docs: Add Track Fitting

[AJPfleger]

Add a documentation for the track fitting and explain a bit KF, (GSF), and (GX2F (wip) - when it is finished).

[codecov]

Codecov Report

Merging #1398 (1b1e9a3) into main (66e0b95) will decrease coverage by 0.87%.
The diff coverage is n/a.

❗ Current head 1b1e9a3 differs from pull request most recent head 49c97a0. Consider uploading reports for the commit 49c97a0 to get more accurate results

@@            Coverage Diff             @@
##             main    #1398      +/-   ##
==========================================
- Coverage   48.64%   47.76%   -0.88%     
==========================================
  Files         381      380       -1     
  Lines       20785    20164     -621     
  Branches     9519     9387     -132     
==========================================
- Hits        10110     9632     -478     
+ Misses       4104     4083      -21     
+ Partials     6571     6449     -122     

Impacted Files	Coverage Δ
...Acts/Utilities/detail/gaussian_mixture_helpers.hpp	17.85% <0.00%> (-51.79%)	⬇️
Core/include/Acts/TrackFitting/detail/GsfUtils.hpp	39.24% <0.00%> (-14.10%)	⬇️
Core/include/Acts/Utilities/Delegate.hpp	77.27% <0.00%> (-9.57%)	⬇️
Core/src/Material/MaterialSlab.cpp	47.36% <0.00%> (-7.18%)	⬇️
Core/include/Acts/Geometry/Extent.hpp	87.50% <0.00%> (-6.95%)	⬇️
Core/include/Acts/Utilities/Result.hpp	72.00% <0.00%> (-6.44%)	⬇️
...clude/Acts/EventData/detail/CalculateResiduals.hpp	50.00% <0.00%> (-5.56%)	⬇️
...cts/EventData/SingleCurvilinearTrackParameters.hpp	44.44% <0.00%> (-5.56%)	⬇️
Core/include/Acts/Surfaces/EllipseBounds.hpp	58.33% <0.00%> (-4.63%)	⬇️
Core/src/Visualization/EventDataView3D.cpp	25.00% <0.00%> (-4.42%)	⬇️
... and 170 more

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[benjaminhuth]

Maybe we can get this in with these information now and add more infos and the GX2F later?