Add Catalyst section

This MR collects all the information and documentation for ParaViewCatalyst in a single place. It adds a new toplevel section "Catalyst" with the following sections

• Getting Started: derived from https://discourse.paraview.org/t/how-to-try-out-catalyst2/9008 with few fixes and additions
• Background section: Copy of https://gitlab.kitware.com/paraview/paraview/-/blob/master/Utilities/Doxygen/pages/ParaViewCatalyst.md
• Blueprints section: Copy of https://gitlab.kitware.com/paraview/paraview/-/blob/master/Utilities/Doxygen/pages/ParaViewCatalystProtocol.md

Actions to take once merging this:

• remove the above markdown files or add a note redirecting here. (paraview!6429 (merged))
• Add catalyst_pipeline_live.py mentioned in the getting started section in https://gitlab.kitware.com/paraview/paraview/-/tree/master/Examples/Catalyst2/CxxFullExample so that the example comes with everything described in the guide. Right now it is an attachment in discourse. (paraview!6429 (merged))
• Add links from examples to here (!66 (merged))