Add help text to project topics selector

What does this MR do and why?

This adds UI text to help users working with topics in the project settings page.

Relates to &8489 and #393263.

Before:

After:

How to set up and validate locally

1. Create a project.
2. Navigate to the project settings page (ie. http://127.0.0.1:3000/dev/example-project/edit).
3. Note the description text below the topics selection input.
4. Click the Learn more. link and verify you are redirected to the topics help page.
5. Add one or more topics to the project and save changes.
6. Remove one or more topics from the project and save changes.

MR acceptance checklist

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