Add an Install on Azure Kuberentes Service doc

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Hi there Knative team! I'm on the Azure Kubernetes Service team! We've been checking out Knative and we're fans 🎊. I've been using Knative on AKS as I've been coming up to speed and it's been pretty nice! I've gone ahead and written up a doc on how to install on Knative on AKS. I'd love to PR the doc, if that's acceptable! 

