Blueprint: Feature Flags usage in GitLab development and operations

What does this MR do and why?

The goal of this blueprint is to make the feature flag process safer, more maintainable, lightweight, automated, and transparent.

Review app: https://main-ee-133018.docs.gitlab-review.app/ee/architecture/blueprints/feature_flags_usage_in_dev_and_ops/index.html

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.