Document CI job activity limit

Related to: #196140 (closed)

What does this MR do?

Add documentation for CI job_activity_limit which is enforced when a pipeline is being created.

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