Build a 'project templates' page from scattered bits
### Problem to solve
We have no centralized information about project templates. It's scattered across several pages. As part of my OKR (https://gitlab.com/gitlab-org/technical-writing/-/issues/839+) let's extract this information and build a single, unified page for it.
Pages with some part of the information:
- https://docs.gitlab.com/ee/administration/custom_project_templates.html (this page will be listed as my OKR)
- https://docs.gitlab.com/ee/user/group/custom_project_templates.html
- https://docs.gitlab.com/ee/user/project/pages/getting_started/pages_new_project_template.html
- https://docs.gitlab.com/ee/user/project/working_with_projects.html#create-a-project-from-a-built-in-template
### Further details
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### Proposal
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### Who can address the issue
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### Other links/references
- Related to https://gitlab.com/gitlab-org/technical-writing/-/issues/839+
- On the [CTRT by writer](https://docs.google.com/spreadsheets/d/1JlARS_KuKPjelPKw6F7uW9vlwt9JQbrtKnz_PYfs2Lg/edit#gid=0) spreadsheet, this page is line 9 when sorted by score.
- [14.09.2023 sync meeting notes](https://docs.google.com/document/d/1TeOiCybU9QScEL4V8HdRJZXqPLrN_Va7T1GmwfEt8x0/edit) includes a video recording
issue