examples: add user-and-acls example

[sago2k8]

Summary

• Adds a focused example at examples/user-and-acls/ that provisions a SASL/SCRAM user and a typical consumer-style ACL bundle (CLUSTER, TOPIC, GROUP — 9 ACLs via for_each over operation lists) against an existing Redpanda Cloud cluster.
• Deliberately does not bootstrap the resource group, network, or cluster — that path is covered by examples/redpanda-role. The new example assumes the cluster already exists and is referenced via var.cluster_api_url.
• README spells out the service-account-with-Admin-role prerequisite, the env-var credential flow, how to fetch the dataplane URL from the Control Plane API, and pointers to the Cloud RBAC docs. Includes outputs, cleanup, and a "Customizing the ACL set" section keyed on the locals operation lists.
• Pins providers explicitly (redpanda-data/redpanda, hashicorp/random) so it runs cleanly in air-gapped / mirror environments. Pins required_version >= 1.11 because the example uses the write-only password_wo attribute.

This grew out of a customer case where the failure surface was unclear; the example exists so customers and support can validate the user/ACL provisioning path against any cluster in one terraform apply.

Test plan

• terraform init && terraform validate from inside examples/user-and-acls/
• terraform apply against a non-prod cluster with a REDPANDA_CLIENT_ID/REDPANDA_CLIENT_SECRET for a service account that has the Admin role — confirms the 1 user + 9 ACLs land
• terraform destroy cleans up without leftover resources
• Re-read the README end-to-end with fresh eyes; confirm links resolve and prereqs are sufficient for someone new to the provider

[gene-redpanda]

Needs to be added to a template so that it can be referenced in the generated docs. The example as currently implemented is a bare directory

[gene-redpanda]

See comment re: adding example to a template