use prettylint for markdown

[SimenB]

Summary

Uses prettylint instead of prettier --list-different.

See difference when invoking them (ignore command in the middle wihout quotes, I hit ^C and was too lazy to clean it up 😅):

(current behavior has already caused confusion: #5648).

Test plan

N/A

[codecov-io]

Codecov Report

Merging #5651 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5651   +/-   ##
=======================================
  Coverage   62.35%   62.35%           
=======================================
  Files         216      216           
  Lines        7908     7908           
  Branches        4        4           
=======================================
  Hits         4931     4931           
  Misses       2976     2976           
  Partials        1        1

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[SimenB]

Should we start having precommit hooks?

[rickhanlonii]

Way better 💯

[github-actions]

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