FAQ Cleanup and Revamp

Started reviewing, cleaning, sorting the hifis.net/faq.

• Tried to identify (and move on top) only the actually often asked questions.
  • Deleted some obivously outdated things
• Reverted to markdown and removed html clutter mostly.
  • some older questions yet to be converted.
• Fix text for "roadmap" Q&A
• "Who can use Helmholtz cloud serivce" -> eine F+A
• fix html proofer error

For later:

• Some would actually better fit to a How-to. If so, we should move them to doc and just link from here.

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More loose topics (maybe for separate MR):

• keycloak cert errors, solution proposed in https://support.hifis.net/#ticket/zoom/8431
• events.hifis.net Role of managers, and their duty to substructure their categories

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• Review here
• Edit here
• Diff to master