fix(gateway): Use Workshop Studio S3 URLs for Cognito Launch Stack button

[gunaven]

Amazon Bedrock AgentCore Samples Pull Request

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Issue number: #1922

Concise description of the PR

Changes to 00-optional-setup/README.md, because the Launch Stack buttons
point to a private staging repo URL that returns 404 for external users,
and CloudFormation rejects raw GitHub URLs with "TemplateURL must be a
supported URL." Replaced with verified Workshop Studio S3 URLs.

User experience

Before: Clicking the "Launch Stack" button results in a CloudFormation error — the templateURL points to raw.githubusercontent.com (via a private staging repo) which CloudFormation does not accept.

After: Clicking the "Launch Stack" button opens the CloudFormation console with the Cognito template pre-loaded from Workshop Studio S3. The stack deploys successfully in both us-east-1 and us-west-2.

Checklist

• I have reviewed the contributing guidelines
• Add your name to CONTRIBUTORS.md
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Are you uploading a dataset?
• Have you documented Introduction, Architecture Diagram, Prerequisites, Usage, Sample Prompts, and Clean Up steps in your example README?
• I agree to resolve any issues created for this example in the future.
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.