NGINX One Console: Add RBAC Info

[mjang]

Is your feature request related to a problem? Please describe

Based on existing functionality from XC, we need to document how NGINX One Console (N1C) can use:

• Custom Roles
  • We currently use "standard" pre-defined roles for admin, user, monitor
• Namespaces
  • We already have a definition in the Glossary (not sure if it's accurate)
  • It's part of the XC Administration functionality, and we need an overview, and a link

Also, each role will have different levels of access to various API endpoints

• The endpoints already exist in our API Reference
• There are additional XC API changes, to be covered in XC docs

Describe the solution you'd like

Potentially:

• API reference changes
  • Reality: probably no change to NGINX API docs
• Concept info, similar to https://docs.nginx.com/nginx-instance-manager/admin-guide/rbac/overview-rbac/
• Release Notes
• Glossary updates

The solution might be straightforward

• To be written by PM
• For details on how to create these custom roles, refer to XC docs at https://docs.cloud.f5.com/docs-v2/administration/how-tos/user-mgmt/roles

Additional context

Target for N1C "release" (of existing functionality): Feb 28