Edit Prettier doc

[ruthemmanuelle]

What does this PR do?

• Edits @ntotten 's new setup doc for Prettier.
• Moves the Prettier doc to the Getting Started section, since it's about setting up Prettier more than about using it.

What issues does this PR fix or reference?

#1208, @W-6003543@

[codecov]

Codecov Report

Merging #1212 into prettier_doc will increase coverage by 0.12%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           prettier_doc   #1212      +/-   ##
===============================================
+ Coverage         70.88%     71%   +0.12%     
===============================================
  Files               196     196              
  Lines              7397    7397              
  Branches            781     781              
===============================================
+ Hits               5243    5252       +9     
+ Misses             1998    1989       -9     
  Partials            156     156

Impacted Files	Coverage Δ
...x-replay-debugger/test/integration/goldFileUtil.ts	97.72% <0%> (+20.45%)	⬆️

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[lcampos]

LGTM