Add code snippet to "Serving with AWS Lambda" showing how to prepare app with Mangum

[cosmicBboy]

The https://unionml.readthedocs.io/en/latest/serving_aws_lambda.html page currently doesn't show how to update your UnionML app for deployment to AWS Lambda.

Add a section near the top of the page that shows how to create a lambda handler from a FastAPI app with Mangum, see here.

The end product should look something like this