docs: Gaussian Sum Filter

[benjaminhuth]

Some high level GSF documentation. Maybe add some figures later...

[codecov]

Codecov Report

Merging #1403 (a0195d4) into main (ff772f1) will decrease coverage by 0.82%.
The diff coverage is n/a.

❗ Current head a0195d4 differs from pull request most recent head 531e85e. Consider uploading reports for the commit 531e85e to get more accurate results

@@            Coverage Diff             @@
##             main    #1403      +/-   ##
==========================================
- Coverage   48.58%   47.76%   -0.83%     
==========================================
  Files         381      380       -1     
  Lines       20651    20164     -487     
  Branches     9476     9387      -89     
==========================================
- Hits        10034     9632     -402     
- Misses       4064     4083      +19     
+ Partials     6553     6449     -104     

Impacted Files	Coverage Δ
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%)	⬇️
Core/include/Acts/Surfaces/TrapezoidBounds.hpp	70.00% <0.00%> (-3.92%)	⬇️
Core/src/Geometry/TrackingGeometryBuilder.cpp	61.11% <0.00%> (-3.89%)	⬇️
... and 156 more

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[benjaminhuth]

Can you re-approve, @paulgessinger ?