Add Agent Control Plane docs (#5939)

* docs: split Agent Control Plane into Overview/Monitoring/Rules and localize

Mirror the secrets-manager folder convention for ACP: one folder per
locale with overview, monitoring, and rules pages. Replaces the two
flat agent-control-plane.mdx / agent-control-plane-rules.mdx files
with a 3-page layout, adds full translations for pt-BR, ko, and ar,
and rewires docs.json to register the new paths under each locale's
Manage group across the same 4 versions where ACP already lived.

* docs: flag Agent Control Plane as Beta in overview pages

Add a Beta callout right after the lead screenshot on the ACP
overview page across en, pt-BR, ko, and ar, matching the convention
used by Secrets Manager.