docs: add cross-references to existing rate limiting design docs
Point readers of the 2022 blueprint and the simplification doc
to the new unified rate limiting architecture document for the
detailed technical design.
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> **Note:** The technical design for Phase 2 (application-level unification) and
> beyond is detailed in the [Unified Rate Limiting Architecture](../unified_rate_limiting/)
> design document. This document remains the high-level roadmap.
## Summary
As GitLab.com has evolved, we've introduced rate limits and throttling measures at many layers to enhance security and performance of our platform, ensuring availability and user satisfaction are at a desired level. These measures have often been reactionary, and we have not had a consistent strategy for defining and enforcing these limits.