Take privacy policy back into effect

[samumantha]

• Hedgedoc instead of HackMD
• Phase 4 instead of dates from phase 3

Review: Does this make sense and touch all topics needed to bring the policy back into effect for now? Be critical!

• It somewhere says that we will work on a new policy, and I think we should still do that, therefore now it "just" needs to be enough for the September workshop.

Reviewers chosen based on previous editors of the document.

Related #940

[github-actions]

Once the build has completed, you can preview your PR at this URL: https://coderefinery.org/previews/PR941/

[samumantha]

Merging this for now to fully open registration next week, it is likely "good enough" for now.
Let's revisit the topic later though