Minor documentation updates after shipping Emoji

[Reinmar]

Special characters - "Adding new category" section

https://ckeditor.com/docs/ckeditor5/latest/features/special-characters.html#adding-a-new-category

The cheapest option would be... another callout saying that there's an official Emoji plugin if someone needs it.

This option actually may make some sense because there's a small chance that there are links to this section outside.

Special characters – just mentioning Emoji plugin

There's some resemblance between these two plugins from the functional perspective. Mentioning that Emoji exists somewhere in the intro would be good IMO.