Docs: Instance Template Repository clarification
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### Problem to solve
Our [documentation on the Instance Template repository feature](https://docs.gitlab.com/ee/user/admin_area/settings/instance_template_repository.html) is unclear and could use some work. The current page is in the `Admin` section and is aimed at administrators. It even shows "Premium" implying this is a self-managed feature only. However, this does extend all the way down to groups and therefore a group owner can designate a template folder and a user is able to use custom templates.
I feel that the wording custom templates makes it seem like you can customize your category type, but this is a [feature request](https://gitlab.com/groups/gitlab-org/-/epics/3597)
### Proposal
I'm not sure how to approach this but ultimately I think we need to clarify that this feature can be used on GitLab.com and it should have some indication of how to approach this from a group level. We have a group-level project template page [here](https://docs.gitlab.com/ee/user/group/custom_project_templates) but this feature is different.
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