Substitution rule: emoji does not take 's' in the plural

What does this MR do?

We originally standardized on emojis, but we're contradicting both the Microsoft and Google style guides in doing so:

• https://developers.google.com/style/word-list#letter-e
• https://learn.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/e/emoticons-emoji

64 hits in the documentation. A few are in subheadings, so rather than complicate this initial MR with potential broken anchors, I'll split the fixes out into a separate MR.

Related issues

Some extra reading:

• Why @annabeldunstone asked: Draft: Added group settings for creating custom... (!124855 - closed)
• Discussion source: https://gitlab.slack.com/archives/C16HYA2P5/p1688744155649419 in #docs.
• https://www.grammarly.com/blog/emojis-emoji/
• @marcel.amirault's discussion about nouns lifted from Japanese
• Related to Update user-facing bad plurals of 'emoji' in th... (!125791 - merged) for frontend fixes
• Related to Clean up bad plural of 'emoji' in docs (!125782 - merged) for the docs fixes

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