[Emoji][String] Extract Emoji from String documentation

[MrYamous]

Give a try for #20114

I've mainly copy/paste current documentation, with following changes :

• change name for internal ref link (remove string part)
• remove a versionadded directive for Inverse Emoji Transliteration (Emoji component was added in 7.1, I think it's not necessary to indicate this feature has been add in 7.1 too)
• remove Gitlab and Slack example for transliterating services emojis as short code and replace it with a note to mention which services are available
• add a sentence about slug emojis as this part stay in String documentation

[javiereguiluz]

Matthieu, thanks a lot for the time taken to extract the docs. While merging I re-added the sections about GitLab and Slack transliteration. I agree that they are a bit repetitive, but showing only the GitHub example would make some folks angry because they prefer other services. Thanks!