Adds Redis configuration to docs

[stevsmit]

Issue: https://issues.redhat.com/browse/PROJQUAY-3888

[syed]

There are 3 possibilities for modelcache

1. memcache (this is the default) you don't need to configure anything
2. single redis instance with optional RO replicas. Config for this:

    DATA_MODEL_CACHE_CONFIG:
      engine: redis
      redis_config:
        primary:
            host: <host>
            port: <port>
            password: <password if ssl is true>
           ssl: <true | false >
        replica:
            host: <host>
            port: <port>
            password: <password if ssl is true>
           ssl: <true | false >
        

Here replica is optional and if you don't specify that all connections will go to the primary

3. clustered redis instance. Config for this is:

    DATA_MODEL_CACHE_CONFIG:
      engine: rediscluster
      redis_config:
        startup_nodes:
          - host: <cluster-host>
            port: <port>
        password: <password if ssl: true>
        read_from_replicas: <true|false>
        skip_full_coverage_check: <true | false>
        ssl: <true | false > 

[LiZhang19817]

/lgtm