[FEAT docs] - Enhance Documentation - IDP config

[uwe-mayer]

Description

This is part of enhancing the Installation Experience of Greenhouse

Acceptance Criteria

• It would be helpful to document the idproxy callback URL that needs to be registered in the IDP
• Documentation for IDP custom claim usage for login_name would be useful (see feat(dex): add option to modify userid claim, skip email_verified verification #1569 for customization)
• It would be worth noting that the group referenced in mappedOrgAdminIdPGroup should already exist in the IDP for RBAC authorization to work