Improve documentation on --no-delete flag

[devshawn]

As many users already have a cluster with topics/ACLs defined, we should provide better documentation for using kafka-gitops against an existing Kafka cluster. This includes improving the documentation around the --no-delete flag and topic blacklist setting.

[gquintana]

As I deleted everything on my dev cluster on my first attempt with kafka-gitops, I would prefer to inverse the flag and get an --allow-delete flag. For safety reasons, to avoid tears, it would be a wise change.