Docs for VSA cycle time and lead time

What does this MR do?

This adds a section to the Value Stream Analytics documentation to explain how the lead time and cycle time metrics are calculated.

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 #217997 (closed)