Add disclaimer for direct Git access

What does this MR do?

Adds a disclaimer for direct Git access due to design decisions in Gitaly and Praefect.

Relates to #223200 (comment 370945597)

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

Low risk, doc only change.