Resources documents do not have an emoji picker. There is no smile icon next to the document name.

Adding an emoji

Type or paste the emoji directly into the document title. On Mac press Control + Command + Space, on Windows press the Windows key + period. To remove one, edit the title and delete the character.

My emojis disappeared after the upgrade

That was the upgrade from Docs to Resources. The upgrade screen has an optional "I want to keep emojis in my titles" checkbox, and titles are cleaned of emoji when it is left unchecked. Add them back by editing the title.

The smile icon belonged to the previous Docs editor and is not part of Resources.