Update Network Secured README.md

[fosteramanda]

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[Copilot]

Pull Request Overview

This PR reorganizes and enriches the private-network-standard-agent-setup README by adding a high-level overview, expanding supported regions, and deepening the architecture and resource sections.

• Introduced a new “Overview” section detailing the IaC solution and its security model.
• Updated and unified the list of supported Class A subnet regions, and propagated changes across the document.
• Refactored numbering in prerequisites, expanded architecture/resource tables, and cleaned up Markdown structure.

Comments suppressed due to low confidence (3)

samples/microsoft/infrastructure-setup/15-private-network-standard-agent-setup/README.md:17

• Use a consistent flag style (e.g., [NEW] or [New]) when marking new regions to ensure uniform formatting throughout the document.

> Class A subnet support is only available in a limited number of regions and requires your subscription id be allowlisted. Please reach out to fosteramanda@microsoft.com if you are interested in getting access. **Supported regions:** West US, East US, East US 2, Central US, Japan East, France Central, [New] Spain Central, [New] UAE North

samples/microsoft/infrastructure-setup/15-private-network-standard-agent-setup/README.md:50

• Prerequisite numbering repeats “1.” for multiple items. Consider using sequential numbers or switch to bullet points (-) for clarity and consistency.

1. **Register Resource Providers**

samples/microsoft/infrastructure-setup/15-private-network-standard-agent-setup/README.md:144

• The list marker +- may render incorrectly in Markdown. Indent sub-items or use proper list syntax (e.g., - with two-space indent) to maintain readability.

- **Create a New (or Use Existing) Resource Group**