docs: document observability-proxy (tenant logs and metrics API) #787

Cozystack is gaining a new system component, observability-proxy, which lets tenant users read logs and metrics of their own namespace and applications with their Kubernetes credentials (design proposals cozystack/community#95 and cozystack/community#94). The implementation PRs: cozystack/cozystack#5214, cozystack/cozystack#5219, cozystack/cozystack#5220, and the console views in cozystack/cozystack#5218.

Per the downstream trigger map in docs/agents/contributing.md, the website needs follow-ups in content/en/docs/next/:

• guides/platform-stack/_index.md: add observability-proxy to the platform components.
• operations/configuration/licenses.md: list the new component.
• operations/configuration/variants.md: the component is part of the system bundle, enabled by default on the full variants and opt-in on the slim ones.
• A user-facing page (for example under the monitoring section) describing:
  • the API roots /apis/cozystack.io/v1alpha1/namespaces/<ns>/{logs,metrics}/ and /apis/apps.cozystack.io/v1alpha1/namespaces/<ns>/<plural>/<name>/{log,metrics}/;
  • that access follows tenant RBAC (view and above), and what the new cozystack.io logs and metrics permissions mean for custom roles;
  • the console Logs page and the Logs/Metrics tabs on applications;
  • the limits (time range, step, result size) and the status codes (401, 403, 422, 429, 502);
  • the release note of the VMAgent scrape-class change: series of application pods get new labels, so graphs show a break at the upgrade.

The pages should land once the implementation PRs are merged; this issue tracks them.