doc(group_theory/coset): Mention "Lagrange's theorem" (#12137)
Suggested here: https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there.20code.20for.20X.3F/topic/.E2.9C.94.20Lagrange's.20Theorem.20.28Group.20theory.29/near/272469211
Co-authored-by: tb65536 <tb65536@users.noreply.github.com>
