Jira app - improve setup required instructions for GitLab.com

What does this MR do and why?

As per issue, it adds additional step for Self-Managed instances.

Screenshots or screen recordings

Before	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 #430044 (closed)