Bicep Example 101 for Backup Vault

**Is your feature request related to a problem? Please describe.**
Hi,

I am trying to create a Bicep template for Backup Vault with a Postgres PostgreSQL policy.

The examples here are not clear eg. datasourceTypes

https://docs.microsoft.com/en-us/azure/templates/microsoft.dataprotection/2021-01-01/backupvaults/backuppolicies?tabs=bicep#backuppolicy

I am also unable to export templates from my Resource Group to cross reference the Backup Vault ARM template. If I had access, I would be able to cross reference ARM to Bicep.

{"code":"ExportTemplateCompletedWithErrors","message":"Export template operation completed with errors. Some resources were not exported. Please see details for more information.","details":[{"code":"ResourceTypeSchemaNotFound","target":"Microsoft.DataProtection/BackupVaults","message":"The schema of resource type 'Microsoft.DataProtection/BackupVaults' is not available. Resources of this type will not be exported to the template."},

**Describe the solution you'd like**
1. An Example 101 for Bicep Vault with policy and retention.
2. The ability to export Backup Vault templates

Start with the linked BackupPolicies Bicep documentation, focusing on the Microsoft.DataProtection/BackupVaults resource and the unclear datasourceTypes example. Define done as a complete Backup Vault example covering policy and retention, plus a decided path for exporting Backup Vault templates; no repository file or test is named in the issue.

Written by the indexing model from the issue text.