docs(kraft): adds a procedure for zookeeper to kraft migration

[PaulRMellor]

Documentation

Adds a new procedure that describes how to migrate from ZooKeeper-based to KRaft-based Kafka cluster.
Also includes steps to roll back, if required.

Checklist

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

• 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
• Supply screenshots for visual changes, such as Grafana dashboards

[ppatierno]

@PaulRMellor I added the "do-not-merge" label because this one should anyway wait for the implementation in #9480 being merged as well.

[scholzj]

LGTM. But it shoud be approved by Paolo as the migration SME.

[see-quick]

first iteration adding a few comments, I would also do the second one to check all procedures you have add here.

[ppatierno]

LGTM! Thanks!
As already said we'll not merge this until the corresponding implementation PR #9480 is merged as well.

[fvaleri]

Thanks. Left some comments.

[ppatierno]

@PaulRMellor while having an offline chat with @fvaleri it could be worth mentioning that if your initial configuration has the inter broker protocol and/or log message format set, you should remove them at the end of the migration. The brokers and controllers will be rolled one more time to update them.
At the same time even when removing the spec.zookeeper.
There are warnings in the Kafka CR but after the rolling, those are removed.

[see-quick]

Just a few comments from me 👍 Looks good

[ppatierno]

@PaulRMellor We should take all the updates not related to migration out of this PR but having a new one for them. I see you updated annotation values in all other places, but I would move them for a new PR. Let's take this PR for migration only.

[ppatierno]

LGTM. Back to green!

[fvaleri]

LGTM. Thanks.

[ppatierno]

@PaulRMellor while having an offline chat with @fvaleri it could be worth mentioning that if your initial configuration has the inter broker protocol and/or log message format set, you should remove them at the end of the migration. The brokers and controllers will be rolled one more time to update them.
At the same time even when removing the spec.zookeeper.
There are warnings in the Kafka CR but after the rolling, those are removed.

@PaulRMellor I don't see addressed the above comment here.