mathlib3

eadd4400 - feat(group_theory/index): `card_mul_index` (#10228)

feat(group_theory/index): `card_mul_index` (#10228) Proves `nat.card H * H.index = nat.card G` as the special case of `K.relindex H * H.index = K.index` when `K = ⊥`. Co-authored-by: tb65536 <tb65536@users.noreply.github.com>