docs(apigatewayv2): revive README.md (#28435)

The readme for apigatewayv2 is currently [removed](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_apigatewayv2-readme.html), and this PR gets it back.

I got the original README.md from this commit (just before it got removed):
https://github.com/aws/aws-cdk/blob/187f67b3430dca7ea96a94c66b18694bce213f03/packages/%40aws-cdk/aws-apigatewayv2-alpha/README.md

I confirmed other two modules (integration and authorizers) have correct READMEs already.

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