[Docs] Enhance Roles Documentation

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**