docs: correct stages-aw networking description

[KSAGlory]

Description

Correct the FAST networking overview to match the current implementation. The two networking options configure VPC firewall rules and subnets through Terraform variables and do not deploy hybrid connectivity.

Fixes #191

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Deployment & Compliance Impact

• Applicable Regimes:
  • US Region Restricted (e.g., Access Policy constraint)
  • FedRAMP Moderate
  • FedRAMP High
  • DoD IL4
  • DoD IL5
  • General / All
• NIST 800-53r5 Controls: None. This documentation correction does not modify control implementations.

Checklist

Code Quality & Reusability

• My code adheres to the Maximize Reusability principle. I have not redefined common elements and have reused existing base configurations and modules where possible.
• I have checked that no existing module or configuration in modules/ or fast/ can be leveraged for this change.
• My code follows the established naming conventions outlined in documentation/naming-convention.md.

Documentation

• I have updated the README.md of the modified module or blueprint.
• I have added/updated documentation for inputs (variables) and outputs. Not applicable because no inputs or outputs changed.

Security

• My change adheres to GCP security best practices and the principle of least privilege.
• I have ensured compliance with the targeted regime (FedRAMP Moderate, FedRAMP High, IL5, etc.).

Testing

• I have tested my changes locally.
• I have included details of my testing in this PR.

Testing Performed

• Ran git diff --check.
• Confirmed the README no longer claims that the networking options deploy VPN connectivity or YAML-based factories.
• Verified that both networking implementations configure firewall rules and subnets through Terraform variables.

No runtime Terraform tests were necessary because this change only corrects existing documentation.