New documentation

[axel-grc]

Pull Request Summary

Documentation

• Added the following documentation files:
  • CodeContribution.md
  • Geometry.md
  • Tutorial.md
  • CustomPhantom.sh
• Added new forbild files:
  • Geometries_forbild.txt.sha512
  • SheppLogan_forbild.txt.sha512
• Added examples from old documentation:
  • ForwardProjection
  • RayBoxIntersection
  • WaterPreCorrection
  • ConjugateGradient
  • 4DRooster
  • AmsterdamShroud
  • DaubechiesWaveletsRegularizedReconstruction
  • ElektaReconstruction
  • MotionCompensationReconstruction
  • TotalVariationRegularizedReconstruction
  • VarianReconstruction
• Removed rtkThreeDCircularProjectionGeometry.dox from Doxygen to integrate it into Sphinx.

To Do

[x] Verify and replace references to Doxygen Geometry documentation with links to the Sphinx Geometry documentation (Axel puis Simon)
[x ] Modernize the code for WaterPreCorrection (Simon)
[x] Add result images to the examples
[x] Integrate the FanBeam 2D example from the old documentation into the ConjugateGradient example (Axel)
[x ] Redirect to the new doc on the old doc website

[SimonRit]

Fixes #642 and #643

[SimonRit]

This is a good start but it will need improvements for readability