Add subgroups to group webhook documentation

What does this MR do and why?

This adds "subgroups" to the documentation for group webhooks to make it clear that hooks will be triggered for subgroups as well.

Subgroup hooks are triggered here: https://gitlab.com/gitlab-org/gitlab/-/blob/5046c036d6870ea29b2fba944483ce17d55582ff/ee/app/models/ee/group.rb#L667-676

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