Apply prettier commit hook to mdx files

Nice first issue for someone: add `mdx` to the list files that get formatted by prettier in the git commit hook. Manually style formatting of text and components is so 2010 🙃

Locate the git commit hook's Prettier file list and inspect how extensions are specified. Add mdx to that list, then verify the hook formats MDX files without changing its existing behavior.

Written by the indexing model from the issue text.