Draft: Capture existence of emoji upload page

What does this MR do?

This merge request was started by adding a stub for uploading emoji through the GitLab UI. It's not currently on doc/user/emoji_reactions.md which means @msedlakjakubowski might not be aware of it?

Either way, sending over to @msedlakjakubowski to figure out what to do next to build out this subheading. All the details I have are here, in this Slack thread.

Thanks, @phikai, for bringing this up.

Related issues

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