Update Gitaly troubleshooting

What does this MR do?

Update the gitaly troubleshooting section to include a scenario where the secrets file isn't the same as the app nodes.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content