Add Jira Connect proxy debugging docs

What does this MR do?

We are still seeing users encounter the Failed to update the GitLab instance error. Despite the efforts of #406650 (closed), customers are still reaching out for assistance.

When users setup the GitLab for Jira app from the Atlassian store and they want to connect their self-managed GitLab instance, they must configure GitLab.com as the Jira Connect Proxy. This creates certain issues for GitLab Support:

1. Support Engineers new to the Jira app do not immediately think to check GitLab.com for logs because if the customer is using a self-managed instance, under normal circumstances why would you check the GitLab.com logs?
2. How do Support Engineers locate the necessary information to relay to the customer in the GitLab.com logs?
3. What can customers provide up front to help us debug the problem?

This MR addresses all of those points.

Related issues

Author's checklist

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Reviewer's checklist

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