Add Praefect as a component to the documentation

What does this MR do?

Add Praefect as a component to the architecture of GitLab.

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

Closes: #35281 (closed)