Docs: Feedback for Use the Control Plane API with Serverless

[jason-da-redpanda]

Issue description

the POST for creating a server is missing the
serverless_cluster field

curl -H 'Content-Type: application/json' \
-H "Authorization: Bearer blah-blah" \
-d '{
   "serverless_cluster": {
        "name": "jason-test",
  "resource_group_id": "26be6123-bd6e-4e34-9268-69fbd74d3fe2",
  "serverless_region": "pro-us-east-1"
   }
  }' -X POST https://api.redpanda.com/v1/serverless/clusters

Updates to existing documentation

Use the Control Plane API with Serverless Page source

Link to Redpanda Slack conversation

No response