Add documentation and fixture for alerts endpoint

[epenet]

Using fixtures provided by @jumpjack in #1481

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.75%. Comparing base (9ca3ba6) to head (d438e3d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1540   +/-   ##
=======================================
  Coverage   88.75%   88.75%           
=======================================
  Files          33       33           
  Lines        2134     2134           
  Branches      218      218           
=======================================
  Hits         1894     1894           
  Misses        175      175           
  Partials       65       65           

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