docs(security): updates content for securing kafka access

[PaulRMellor]

Documentation

Updates from review and edit of the content related to security to make it easier to understand and find the relevant information.

• Removes repeated content, including for the following types of content
  • Certificate creation (present in a couple of places)
  • Configuring Kafka brokers and users - now present in a single example
• Restructuring:
  • Setting up client access to a Kafka cluster now has no content related to securing access
  • Setting up secure client access example moves to securing access section
  • Network policies concepts moved to the network policies procedure
  • OAuth 2.0 moves into a separate section
  • Moves user quotas into separate section for better visibility
• Cleanup: the following files have been removed to reduce redundancy and repetition (content absorbed or removed if replicated elsewhere)
  • Security options for Kafka (assembly-securing-kafka-brokers.adoc) -- not required
  • Securing access to Kafka brokers (assembly-securing-kafka.adoc) -- content moved
  • Securing user access to Kafka (proc-configuring-secure-kafka-user.adoc) -- content in example
  • Securing Kafka brokers (proc-securing-kafka.adoc) -- content in example

NOTE: OAUth content is subject to a separate review

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• [ ] Write tests
• [ ] Make sure all tests pass
• [x] Update documentation
• [ ] Check RBAC rights for Kubernetes / OpenShift roles
• [ ] Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• [ ] Reference relevant issue(s) and close them after merging
• [ ] Update CHANGELOG.md
• [ ] Supply screenshots for visual changes, such as Grafana dashboards

[PaulRMellor]

Thanks for the review @scholzj . I addressed all the comments, but this one: https://github.com/strimzi/strimzi-kafka-operator/pull/10071#discussion_r1592693604

As mentioned in the reply, we use the "(Kubernetes only)" in a few places because of downstream doc where the ingress procedure is left out.