feat(FormalConjecturesForMathlib): Introduce 𝔽 notation

[jeangud]

Description

This PR proposes to introduce a notation shorthand for 𝔽-p groups, which is already used (and redefined each time) in files over the repo.

Note: to be merged before #4305 and #4309

Testing

✅ Built the whole repo successfully

$ lake --wfail build
Build completed successfully (8814 jobs).

[github-actions]

👋 This is an automated welcome message. 🤖
Thanks for the contributions!

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• Please take a look at the style guidelines,
  especially the conventions for references, categories, AMS tags, and answer(sorry).
• You can manage some PR labels by leaving a comment with +label-name or -label-name; for example, +awaiting-author or -awaiting-author.
• This repository is mainly for formalised statements. Proofs longer than about 25-50 lines are usually out of scope; longer proofs are welcome to be included/linked via the formal_proof mechanism.

Thanks again for helping improve Formal Conjectures.

[mo271]

Thanks, LGTM, just one suggestion