Documentation: Missing guidance when deploying with an existing Firewall.

### Documentation Issue

Hello,
Some guidance, specially parameters (for example DNS),are missing for the following case, which is common:

- Another team (not data team) deploys a Azure Landing, which looks like this: https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/ready/landing-zone/
- This Landing Zone has already a connectivity subscription, which has firewall, dns, etc. (the data team deployment cannot change those resource)
- The Landing Zone deploys to the data team a Subscription with a Vnet peered to the connectivity subscription

Data team, needs to deploy a smaller version of the Data Management Zone ( Purview and DNS?, etc.).
No guidance and exists, we have been trying for 3 days, and no success yet, mainly DNS issues we guess.

We think this is common scenario if you are following: https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/ready/landing-zone/

Can you please explain a bit about this case?

Thanks!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/Azure/data-management-zone/blob/main/CODE_OF_CONDUCT.md)

Start with the linked Azure Landing Zone guidance and the repository's existing Data Management Zone deployment documentation. Document the parameters and deployment steps for a data team using a peered VNet with an externally managed connectivity subscription, firewall, and DNS; done means the existing-firewall scenario and the smaller Purview-focused deployment are explained, including DNS considerations.

Written by the indexing model from the issue text.