feat: add user management documentation for InfluxDB Cloud Dedicated Admin UI

[MeelahMe]

Closes #6207

Overview

Adds comprehensive documentation for the new user management capabilities in the InfluxDB Cloud Dedicated Admin UI, enabling self-service user administration.

Changes

• Add detailed guide for inviting users through the Admin UI
• Document role-based permissions (Admin, Member, Auditor)
• Include invitation management workflows (send, revoke, view status)
• Explain current limitations and alternative methods
• Follow InfluxData documentation standards and patterns

Impact

• Reduces support requests for basic user management operations
• Enables customers to manage users independently through the Admin UI
• Provides clear guidance on role assignments and permissions

Resolves #6207