style: Run Prettier on docs/ folders

[samestep]

As I mentioned in #899 (comment), we haven't been running Prettier on any folders named docs/ due to a bug in our .prettierignore. This PR fixes that, and also adds packages/core/docs/ to .gitignore.

[codecov]

Codecov Report

Merging #900 (8175278) into main (cf8cf79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #900   +/-   ##
=======================================
  Coverage   67.79%   67.79%           
=======================================
  Files          62       62           
  Lines        8120     8120           
  Branches     1769     1769           
=======================================
  Hits         5505     5505           
  Misses       2608     2608           
  Partials        7        7           

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