Doc using structured transforms with IHttpForwarder

[Tratcher]

https://microsoft.github.io/reverse-proxy/articles/direct-forwarding.html
https://microsoft.github.io/reverse-proxy/articles/transforms.html

Both of these docs talk about transforms, and direct forwarding, but only in the context of deriving from HttpTransformer to do your own custom thing.

ITransformBuilder.Create(Action<TransformBuilderContext> action); as added to help create a transformer for IHttpForwarder while still using all of the structured transforms. This should be covered in both of those docs.
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