Migration FAQ for ARR

Some FAQ's from customers coming from ARR:

preserveHostHeader - https://microsoft.github.io/reverse-proxy/articles/transforms.html#requestheaderoriginalhost

reverseRewriteHostInResponseHeaders - not directly supported - https://github.com/microsoft/reverse-proxy/issues/1548

arrResponseHeader - N/A - YARP doesn't add an X-Powered-By header

minResponseBuffer - not configurable - https://github.com/microsoft/reverse-proxy/blob/25c9ff231afac1b9114028680c344ddc13dc05cd/src/ReverseProxy/Forwarder/StreamCopier.cs#L20-L21

minFreeThreads - https://learn.microsoft.com/en-us/dotnet/api/system.threading.threadpool.setminthreads?view=net-7.0

minLocalRequestFreeThreads - N/A

relaxedUrlToFileSystemMapping - N/A

executionTimeout - ActivityTimeout is the closest approximation - https://microsoft.github.io/reverse-proxy/articles/http-client-config.html#httprequest