Rate limitations announced for Projects, Groups, and Users APIs

Introduces rate limitations for Projects, Groups, and Users APIs to address increased server load and performance impacts. Details the rollout schedule, including brownouts and permanent implementation. Explains the feature flag strategy for GitLab.com vs. Self-managed/Dedicated instances, with default settings for new installations. Provides specific rate limits per endpoint for authenticated (user-based) and unauthenticated (IP-based) requests, and outlines the '429 Too Many Requests' response for exceeding limits.