docs: add `mdformat-gfm-alerts`

[KyleKing]

Add link to a new plugin for mdformat-gfm-alerts

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (782cb22) to head (80d4823).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #427   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          805       805           
=========================================
  Hits           805       805           

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[KyleKing]

Bump, is there someone who could review and approve trivial documentation PRs like this one, #418, #424, and #425?