Add documentation on how to customize incident actions

[elfjes]

Scope and purpose

Add some documentation on how to customize incident (bulk) actions. This also makes it explicit that the INCIDENT_UPDATE_ACTIONS registry is part of Argus' public api

Contributor Checklist

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More information about contributing to Argus can be found in the
Development docs.

• Added a changelog fragment for towncrier
• Added/changed documentation
• 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

[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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.52%. Comparing base (bdf2d38) to head (0f5091e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1212   +/-   ##
=======================================
  Coverage   77.52%   77.52%           
=======================================
  Files         141      141           
  Lines        5505     5505           
=======================================
  Hits         4268     4268           
  Misses       1237     1237           

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