[DOCS] Document how to use Cluster CDK for testing

[dbwiddis]

Is your feature request related to a problem?

The opensearch-cluster-cdk project makes spinning up an OpenSearch cluster for testing very easy. Unfortunately it is currently configured to only work for releases so some code changes must be done for it to work with snapshots.

Additionally, some extra work is needed to spin up an extension node.

What solution would you like?

Document the additional steps required to spin up a cluster for testing.

What alternatives have you considered?

I'm submitting some changes to the CDK project to make some of these things easier, but the sequence of setup should still be documented.

Do you have any additional context?

https://github.com/opensearch-project/opensearch-cluster-cdk/issues/26 https://github.com/opensearch-project/opensearch-cluster-cdk/issues/27 https://github.com/opensearch-project/opensearch-cluster-cdk/issues/28 https://github.com/opensearch-project/opensearch-cluster-cdk/issues/29