docs: clarify that domainName is optional with auto-generated defaults

[konokenj]

Problem

The current documentation and code comments don't clearly explain why domainName is optional and what happens when it's not provided. Users might assume that a custom domain is required for the infrastructure to work, especially for Cognito authentication and CloudFront distribution.

Solution

Added comprehensive documentation comments throughout the codebase to clarify that:

1. For Cognito: When no custom domain is provided, a random prefix is automatically generated to create a unique Cognito domain (e.g., webapp-abc123def4.auth.us-west-2.amazoncognito.com)
2. For CloudFront: When no custom domain is provided, CloudFront's default domain is used (e.g., d1234567890.cloudfront.net)
3. The infrastructure is fully functional without a custom domain setup

Changes

Updated documentation comments in:

• cdk/bin/cdk.ts - EnvironmentProps.domainName
• cdk/lib/main-stack.ts - MainStackProps
• cdk/lib/us-east-1-stack.ts - UsEast1StackProps
• cdk/lib/constructs/auth/index.ts - AuthProps and inline comments
• cdk/lib/constructs/webapp.ts - WebappProps
• cdk/lib/constructs/cf-lambda-furl-service/service.ts - CloudFrontLambdaFunctionUrlServiceProps

These improvements make it easier for users to understand that the custom domain is truly optional and how the system behaves in both scenarios.

[tmokmss]

Thanks!