Doc using structured transforms with IHttpForwarder

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 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.
https://github.com/microsoft/reverse-proxy/commit/2f2fc26d258f1b6f019650ed87414ab7c22ff721#diff-864e9f675b0f53707e74be0e99181f657646b2b33a6f75f04f94b3715181ffc1R44-R50