Update Praefect docs with new connection tester subcommand

What does this MR do?

This MR also Praefect's documentation to mention proper usage of the new diagnostic tool.

Closes gitaly#2259 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

This is a low risk change since the new code in Gitaly-Praefect is an auxiliary command for troubleshooting.