Add "Enable pipeline suggestion banner" setting

What does this MR do and why?

In #337560 (closed), we want to give administrators the ability to disable showing pipeline suggestion banner in merge requests with no pipeline.

In this MR, a new setting ("Enable pipeline suggest banner") is added at the end of Admin Area > Settings > CI/CD page as shown below:

This new setting is a boolean in the database which defaults to true to be backwards-compatible with the current behavior (of (suggestion banner is shown in merge requests with no pipeline).

What's in this MR

• Code and tests to implement addition of new setting to Admin Area > Settings > CI/CD page
• Docs for the new setting

What's not in this MR

• Code to update the logic to show pipeline suggestion banner in merge requests with no pipeline: 👉🏼 !71015 (merged)

How to set up and validate locally

1. Login as an administrator
2. On the top bar, select Menu > Admin
3. On the left sidebar, select Settings > CI/CD
4. Expand Continuous Integration and Deployment
5. Confirm that Enable pipeline suggestion banner checkbox is checked
6. Uncheck Enable pipeline suggestion banner checkbox
7. Click on Save changes
8. Confirm that Enable pipeline suggestion banner checkbox is unchecked

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.