Large update of PowhegControl docs

Jan Kretzschmarrequested to merge
jkretz/athena:jk_update into 21.6

This is a major overhaul of the ATLAS PowhegControl documentation:

  • adapted main documentation to current situation where generation should be in 21.6/22.6
  • fixed some longer-standing 'TODO' items left over from svn&afs -> git&cvmfs migration
  • added some process-specific 'experience notes' and links to a variety of processes

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