Should add a section to the docs for third-party examples and guides using Mangum

I am using Graphene with the Flask graphql integration. Graphql core supports an argument in its execute() function called execution_context_class, among other things as well. I would like to pass a custom ExecutionContext class but it doesn't look like there is a way to do this here in the Flask server integration (and probably the other ones as well).

My use case is that I'd like to use