Add new page "Setup => Caches" to discuss Memcache/Redis configuration

Created by: totten

Memcache and Redis are useful ways to improve performance. @eileenmcnaughton brought this up in https://github.com/civicrm/civicrm-dev-docs/pull/539 , but it feels more like a sysadmin issue. There doesn't seem to be any coverage of this material in the current docs, so I've carried the idea over here and expanded the content.