reorder Articles drop down menu (#156)

_pkgdown.yml

@@ -1,3 +1,28 @@
+navbar:
+  title: ~
+  type: default
+  left:
+  - icon: fa-home
+    href: index.html
+  - text: Reference
+    href: reference/index.html
+  - text: Articles
+    menu:
+    - text: Install the Azure ML SDK for R
+      href: articles/installation.html
+    - text: Set up an Azure ML workspace
+      href: articles/configuration.html
+    - text: Train and deploy your first model with Azure ML
+      href: articles/train-and-deploy-to-aci/train-and-deploy-to-aci.html
+    - text: Train a TensorFlow model
+      href: articles/train-with-tensorflow/train-with-tensorflow.html
+    - text: Hyperparameter tune a Keras model
+      href: articles/hyperparameter-tune-with-keras/hyperparameter-tune-with-keras.html
+    - text: Deploy a web service to Azure Kubernetes Service
+      href: articles/deploy-to-aks/deploy-to-aks.html
+  - text: News
+    href: news/index.html
+
 reference:
 - title: Installation
   desc: