Add How to review for Product Intelligence changes to docs

Add a section in Product Intelligence docs to describe how to review for Product Intelligence changes

Currently we have set up Danger Bot to alert and mention @gitlab-org/growth/product-intelligence/engineers for doing Product Intelligence review

Would be great to have how we do reviews what should we check, for both backend and frontend.

This is not only about Usage Ping metrics

Product Intelligence Review

• Correct Location of metric in Usage Ping
• Correct product_section, product_stage, product_group, product_category is included
• Correct YAML metric defintion
• Metrics name
• Metric description
• Updated Metric Dictionary
• Time frame of the data
• Redis tracking, having or not a feature flag
• Redis tracking, slots, aggregation.
• approve MR
• add product intelligenceapproved label

cc @gitlab-org/growth/product-intelligence/engineers