Revise UI to better explain file templates

What does this MR do and why?

Updates the UI text to better explain what a file template repository is:

• Settings → General → Templates (for groups)
• Admin Area → Settings → Templates (for instances. The text needs to be consistent with whatever we say in Groups)

I have comments and notes; I'll add those separately in threads.

Closes #330161 (closed)

Screenshots or screen recordings

The group page, before:

The group page, after:

The instance page, before:

The instance page, after:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #330161 (closed)