docs(stages,blueprints): document 3-security KMS opt-in behavior and update postgresql PSA requirements - #208

Description

This pull request updates documentation for FAST Stage 3-security and the PostgreSQL blueprint:

• Fixes [Documentation] il5/postgresql README documents cleanup of resources that no longer exist in main.tf; missing PSA-peering prerequisite #116: Updates the PostgreSQL blueprint README Requirements with Private Service Access (PSA) peering prerequisites and replaces stale provider-bug cleanup instructions with accurate module details.
• Fixes [Documentation] 3-security creates no KMS keyrings/keys by default and ships no sample tfvars — document the opt-in behavior #118: Documents the opt-in KMS keyring/key creation behavior in Stage 3-security README, adds a representative terraform.tfvars.sample, and documents key output format.

Fixes #116
Fixes #118

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: SC-12, SC-13

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.

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

• Verified README links and formatting.
• Verified terraform.tfvars.sample syntax and schema alignment with variables.tf.