[Docs] Enhance Roles Documentation

[zihanKuang]

This issue aims to improve the "Roles" documentation to provide clear and complete guidance for users.

• Roles: General overview and purpose of roles. - What happens if the "User" role is removed from an account?
• Assign User Roles: How to assign existing roles to users. What does it mean if an account has a role in one organization, but not another organization?
• Assign Keychains to Roles:
• Create Custom Roles
• Update Custom Roles
• Delete Custom Roles
• Export Custom Roles
• User-defined Roles