[DOC] update the Using the User Operator section

[PaulRMellor]

Signed-off-by: prmellor pmellor@redhat.com

Documentation

The presentation of the content in the_Using the User Operator_ section of the Using Guide was a little confusing.

In this PR, I've reworked to minimize the content and reflect a logical order.
The Operators are introduced earlier in the guide, so this section is just about using the operator to manage users.

Removed

• 4 files
• Replication of 2 files (using x-refs to other locations instead)
• using -uo context as no longer needed

Old structure
6. Using the User Operator
6.1. Mutual TLS authentication
6.1.1. When to use mutual TLS authentication for clients
6.2. Creating a Kafka user with mutual TLS authentication
6.3. SCRAM-SHA authentication
6.3.1. Supported SCRAM credentials
6.3.2. When to use SCRAM-SHA authentication for clients
6.4. Creating a Kafka user with SCRAM SHA authentication
6.5. Editing a Kafka user
6.6. Deleting a Kafka user
6.7. Kafka User resource
6.7.1. Authentication
6.7.2. Authorization
6.7.3. User quotas

New structure
6. Using the User Operator
6.1. Securing user access to Kafka
6.1.1. User authentication
6.1.2. User authorization
6.1.3. User quotas
6.2. Configuring a Kafka user

Checklist

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

• Update/write design documentation in ./design
• Write tests
• Make sure all tests pass
• 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

[strimzi-ci]

Can one of the admins verify this patch?
Run tests: @strimzi-ci run tests

[scholzj]

One nit about empty line. But LGTM otherwise.

[d-laing]

Changes look good. I left a couple of minor comments.

[PaulRMellor]

@ppatierno any further comments?