Document AI Governance policy-change events in Activity logs (follow-up to #25239)

Summary

Follow-up to #25239. That PR documents the sbx-side local audit log (sandboxd's JSONL records of policy decisions + session lifecycle, collected by SIEM tools). This issue tracks the separate, Hub-side audit surface: AI Governance policy-change events in Activity logs, which are already live but undocumented.

Background

Two distinct audit surfaces exist for AI Governance:

• Local audit log (sbx) — what a sandbox policy decided at runtime (allow/deny per request). Documented in #25239 at content/manuals/ai/sandboxes/governance/audit.md.
• Activity logs (app.docker.com) — who changed a governance policy in the Admin Console / via the Governance API. Served by the Docker Audit Logs API and shown in the Activity logs UI.

The governance events are confirmed live (seen in a test org), for example:

• created governance policy
• created / updated / deleted governance policy rule '<name>'
• set governance setting '<name>' from '<old>' to '<new>'

But content/manuals/admin/activity-logs.md currently only documents Settings Management "Policy created/updated/deleted" events — nothing for AI Governance.

What remains

1. Add AI Governance events to content/manuals/admin/activity-logs.md — a new subsection (or rows under Organization events) covering governance.policy.create / update / delete, including the data fields (org_uuid, policy_id, policy_type, rule_changes, settings_changes) and action_description examples.
2. Verify the Hub API reference (reference/api/hub, audit-logs tag) lists the governance.* action types. This is generated/vendored, so a missing entry may need an upstream fix rather than a docs edit.
3. Cross-reference the two surfaces — once the Activity Logs section exists, link it and the sbx audit log page to each other (runtime policy-decision audit ↔ org-level policy-change audit), so readers don't conflate them.

References

• Field descriptions: https://github.com/docker/audit-log-helper/blob/main/docs/field_descriptions.md
• Docker Audit Logs API: /reference/api/hub (audit-logs tag)
• Local audit log page introduced in #25239