[Docs RFC] Add docs for RBAC - viewer (read-only user)

[atovpeko]

PRD: https://docs.google.com/document/d/1oS695PAbaXkBbp5frXDtBY7BZpPuQTD3eBkNeaN_-4s/edit?tab=t.0

Proposed updates for the phase one of RBAC (when it goes full GA, since the beta is private):

• Update this page to include the viewer (read-only) role, including what features are available/not available . This same page will include the description of all the roles in phase 2, possibly with a detailed description of database vs. UI roles.
• Update this page to show how to create a viewer role from the UI. Explain that the viewer roles for service management (control plane) and postgresql (data plane) are different things, and need to be created separately.
• Update the pricing page for all plans to include "Role-based access". When advanced RBAC is rolled out, we can separate "Basic RBAC" and "Advanced RBAC".

[billy-the-fish]

Nice, approved.