Add to development documentation on production Redis usage

https://docs.gitlab.com/ee/development/redis.html currently has some minimal Redis documentation. It doesn't mention much about GitLab.com specific concerns, though. At minimum, we should mention:

1. What's available in structured logging for web requests and Sidekiq jobs (i.e. what we used for #418 (closed)).
2. How to find the Redis slowlog in structured logs (what we used for #416 (closed)).
3. How to perform analysis of the entire Redis keyspace (what we used for #419 (closed)).
4. That we use different instances as described at https://docs.gitlab.com/omnibus/settings/redis.html#running-with-multiple-redis-instances
5. Current uses of Redis that might be interesting, such as the RepositorySetCache, the RepositoryHashCache, and our use of HyperLogLog.