Automatically get section and stage based on group

What does this MR do and why?

Instead of asking the user to input section, stage and group we now only require the group argument.

The provided group is validated against https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/stages.yml from where section and stage is also extracted.

As you can see in the updated documentation, you can now call the generator like this

bundle exec rails generate gitlab:analytics:internal_events \
--time_frames=7d 28d \
--group=project_management \
--event=project_created \
--unique=user.id \
--mr=https://gitlab.com/gitlab-org/gitlab/-/merge_requests/121544

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