[stable31] feat: Save a checksum for documents and use it to detect conflicts

[backportbot]

Backport of #7677

Todo

• Review and resolve any conflicts
• Remove all the empty commits
• Amend HEAD commit to remove the line stating to skip CI

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.69%. Comparing base (8a71954) to head (a4720c9).
⚠️ Report is 4 commits behind head on stable31.

Additional details and impacted files

@@            Coverage Diff            @@
##           stable31    #7724   +/-   ##
=========================================
  Coverage     50.69%   50.69%           
=========================================
  Files           292      292           
  Lines         42331    42331           
  Branches        875      875           
=========================================
  Hits          21458    21458           
  Misses        20752    20752           
  Partials        121      121           

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