Documentation update - Preview services/features used in ALZ

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### Description

#### Is your feature request related to a problem?

Custom roles are using in 'assignable scope' management group, and this is currently in preview based on official doc (https://learn.microsoft.com/en-us/azure/role-based-access-control/custom-roles). Do we have somewhere list of preview features/configurations that are used by this terraform module?

#### Describe the solution you'd like

Documented configurations and feature that is currently in preview that are used by this module.

#### Additional context

Customers avoid to use preview features in production environment.