Add documentation on using Emoji Shortcodes in Markdown text fields.
Add documentation on using Emoji Shortcodes in Markdown text fields.

This updates the Markdown documentation on what Emoji Shortcodes are and
how to use them. It lists a few examples of common ones that people
might want to use and then points to a well-maintained cheat sheet of
GitHub Emoji shortcodes (GitHub, sadly, does not provide their own).

Testing Done:
Built the docs and looked through them. No build errors, and links worked.

Reviewed at https://reviews.reviewboard.org/r/9309/

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