feat: add detailed steps for PostgreSQL mirroring with network isolation enabled

[Harmanpreet-Microsoft]

Purpose

This pull request adds detailed documentation for deploying PostgreSQL mirroring in a network-isolated environment. The new section provides step-by-step instructions for setting up and connecting to a PostgreSQL instance when both Key Vault and PostgreSQL are behind private endpoints, using a VNet-connected jumpbox and a Fabric VNet Data Gateway.

Key additions and improvements:

Network Isolation Deployment Instructions:

• Added a comprehensive guide for scenarios where postgreSqlNetworkIsolation = true, including recommended deployment parameters and a step-by-step process for connecting securely via a jumpbox VM.

Jumpbox Setup:

• Included instructions for installing required tools (Azure CLI, PostgreSQL client, Visual C++ Runtime) on the jumpbox VM to enable secure operations from within the VNet.

Database Preparation:

• Provided PowerShell commands for retrieving the PostgreSQL admin password from Key Vault, creating the seed table, and verifying configuration such as wal_level.

Fabric VNet Gateway Configuration:

• Added steps for creating a delegated subnet, deploying a Virtual Network Data Gateway in Fabric, and connecting it to PostgreSQL using the gateway.

Automation Support:

• Documented how to persist the Fabric VNet gateway ID for use in future automated deployments.
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Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

Verify that the following are valid

• ...

Other Information