Start Praefect config docs

What does this MR do?

Starts documentation for Preafect in omnibus

Part of gitaly#1895 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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