Add readiness review for cache in redis-cluster

What

Readiness Review doc for moving our cache Redis database to Redis Cluster.

Readiness Review issue: #75 (closed)

The readiness review references the existing https://gitlab.com/gitlab-com/gl-infra/readiness/-/blob/master/redis-cluster/ratelimiting.md updated to reflect the cache workload which has a lower failure tolerance and higher load compared to rate-limiting.

Why

Provides horizontal scalability.

Implementation epic: &878 (closed)