docs: document violation acknowledgement

[dottorblaster]

We currently support acknowledging WorkloadPolicy violations (via
kubectl runtime-enforcer policy ack and the status.acknowledgedViolations
field of the WorkloadPolicy CRD), but we have no user-facing documentation
explaining the feature.

The only material available today is auto-generated reference output
(docs/kubectl-plugin/runtime-enforcer_policy_ack.md and docs/crd.adoc),
which documents flags and API fields but not the concept or the workflow.

We should add documentation covering:

• What acknowledging a violation means, and what it does not do (it does not
  change the policy or allow the process going forward).
• The workflow: how to list violations, pick a violation ID, and acknowledge it,
  including the optional --reason and --dry-run flags.
• How acknowledged violations are represented in the WorkloadPolicy status
  (status.acknowledgedViolations), including reason and acknowledgedAt.
• How acknowledgement interacts with violation liveness/retention (see
  docs/rfc/0008-violation-liveness.md) and how many records are kept.
• RBAC/permissions needed to acknowledge a violation.
• A short end-to-end example.