Document the Cloud Connector key rotation order

What does this MR do?

Rotating the Cloud Connector key has no product documentation. The AI Gateway caches the instance public keys, so running rotate right after create, as the create output suggests, makes it reject tokens signed with the new key. The new task restarts the gateway in between, and can move to a general GitLab Duo Self-Hosted page.

Rotation order

sequenceDiagram
  accTitle: Cloud Connector key rotation without authentication failures
  accDescr: The AI Gateway loads the new key before Rails signs with it, and the old key is removed only after tokens signed with it expire.
  participant Admin as Administrator
  participant Rails as Puma, Sidekiq (CachingKeyLoader)
  participant JWKS as GitLab /oauth/discovery/keys
  participant GW as AI Gateway, GitLab Duo Agent Platform service
  Admin->>JWKS: keys:create publishes the new key
  Note over Rails: still signs with the old key
  Admin->>GW: restart
  GW->>JWKS: fetch, caches old and new
  Admin->>Rails: keys:rotate
  Note over Rails: running processes keep the old key
  Admin->>Rails: restart puma, then restart sidekiq
  Rails->>GW: token signed with the new key, accepted
  Note over Admin: wait one hour, the token lifetime
  Admin->>JWKS: keys:trim removes the old key
  Admin->>GW: restart
  GW->>JWKS: fetch, caches the new key only

On GitLab 19.4.1, with a GitLab Duo Workflow token requested every 10 seconds, this order had no authentication failures.

Related issues

Related to #616174.

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An AI coding assistant wrote this change, this description and my replies on this merge request. The assistant also ran each described behavior on a test VM running a GitLab 19.4.1 Linux package instance with an offline cloud license and a self-hosted AI Gateway, checked each claim against those runs and the GitLab, AI Gateway and gitlab-cloud-connector source, and ran the project's Vale and markdownlint on the changed pages.