Restructure projects template topics

What does this MR do?

Seeks to improve documentation for built-in templates (and project templates generally).

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55834.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9204.

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9204), CE-MR afterwards