Add rudimentary docs for planned maintenance

[johannaengland]

Scope and purpose

Good to have some docs for the new planned maintenance functionality.

Contributor Checklist

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Development docs.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation, including updates to the user manual if feature flow or UI is considerably changed
• Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See our how-to
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after
• If this results in changes to the database model: Updated the ER diagram

[github-actions]

Test results

    4 files    520 suites   19m 57s ⏱️
  622 tests   621 ✅ 1 💤 0 ❌
2 488 runs  2 484 ✅ 4 💤 0 ❌

Results for commit e4d240b.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.68%. Comparing base (6297e96) to head (e4d240b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1642   +/-   ##
=======================================
  Coverage   79.68%   79.68%           
=======================================
  Files         132      132           
  Lines        6113     6113           
=======================================
  Hits         4871     4871           
  Misses       1242     1242           

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[aleksfl]

Looks good in general, just some grammar and spelling stuff.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud