chore(group_theory/specific_groups/*): new folder specific_groups (#7018

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This creates a new folder `specific_groups` analogous to `analysis.special_functions`. So far, I have put `cyclic` (split from `order_of_element`), `dihedral`, and `quaternion` there.

Related Zulip discussion: 
https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/group_theory.2Especific_groups