docs: Redis Enterprise 8.0 integration with ACP - #833

Publish the integration guide produced by the Redis Enterprise 8.0
certification on ACP 4.1 (CERT-516): air-gapped image mirroring,
Operator install via bundle YAML, REC and REDB creation covering dense /
sparse / OSS-cluster-compatible placement, verification, upgrade path,
and the known limitations found during validation.

Sourced from the certification materials, redacted for publication —
registry addresses, credentials, and internal environment details are
placeholders.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

- Add the image pull Secret step: the Operator Deployment runs with
  imagePullPolicy: Always and the bundle carries no imagePullSecrets, so
  an authenticated private registry needs one on both the Deployment and
  the REC (spec.pullSecrets).
- Add the license step: REC spec.licenseSecretName with the license under
  the "license" key, and note that it is mutually exclusive with
  spec.license and that omitting both falls back to the trial license.
- Rename the <license-email> placeholder to <admin-username>: REC
  spec.username is the cluster administrator login, not a license field.
  Clarify that the Operator generates the password, not the username.
- Use an 8.0 example for REDB spec.redisVersion and mention the major /
  latest channels.
- State that bundle.yaml v8.0.2-6 references only the Operator image, and
  where the other three mirrored images are consumed.
- Drop "and webhook YAML" from the upgrade path: the admission controller
  is a container in the Operator Deployment.
- Add the missing helm repo add / update before helm install.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>