Add optional billing budgets for AW folder and updated associated docs.

[Calvin-Cheng1]

Description

This PR introduces the ability to configure optional billing budgets for the top-level folder of a landing zone created via the 0-bootstrap stage in fast/stages-aw. This allows for automated cost monitoring and alerting for all projects within the landing zone folder hierarchy.

Changes

• New Feature: Added a billing_budget_amount variable to the 0-bootstrap stage, allowing users to specify a budget amount and optional threshold rules
  (defaults to 50%, 75%, and 90%).
• Implementation: Created fast/stages-aw/0-bootstrap/budgets.tf which uses the billing-account module to create the budget.
• Documentation:
  • Updated the stage's README.md with a new "Billing Budgets" section.
  • Added the new variable to the documentation's input table.
  • Included a commented-out example in terraform.tfvars.sample for easier onboarding.

Fixes #5

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: (If this PR helps satisfy or modifies control implementations, list them here)

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.
• [X I have ensured compliance with the targeted regime (FedRAMP High, IL5, etc.).

Testing

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

Testing Performed

Local terraform apply