Renames and nav tweaks

* Move Go to Production at the end

Nav renames:
* Ubuntu Core Stacks -> Build a stack
* Stack Items -> Features

The main purpose of the rename is to provide a visible path from having an empty board to a device ready for production:
Build a device -> Build a stack -> Go to production

By making "Build a stack" an action to enable features on the device, it give a better mental map of the entire process, and immediately puts the Ubuntu Core Stacks name in context when the user sees it as the title of the Introduction page.

en/metadata.yaml

@@ -30,47 +30,47 @@ navigation:
       - title: Interfaces
         location: guides/build-device/interfaces.md
 
-  - title: Go to production
+  - title: Build a stack
 
     children:
       - title: Introduction
-        location: guides/go-to-production/intro.md
-
-      - title: Simple - Ubuntu store
-        location: guides/go-to-production/simple.md
-
-      - title: Advanced - Branded store
-        location: guides/go-to-production/advanced.md
-
-#  - title: Ubuntu Core Stacks
-
-#    children:
-#      - title: Introduction
-#        location: stacks/index.md
+        location: stacks/index.md
 
 # TODO:
 #   * Need Markdown documentation for location-service
 #   * Need Markdown documentation for media-hub
 #   * Need Markdown documentation for upower
 #   * Need to move the wifi-ap documentation from Google Docs to Markdown
-#      - title: Stack Items
-#        children:
-#          - title: Audio management
-#            location: stacks/audio/index.md
+      - title: Features
+        children:
+          - title: Audio management
+            location: stacks/audio/index.md
 #          - title: Bluetooth management
 #            location: stacks/bluetooth/doc/overview.md
-#          - title: Disk management
-#            location: stacks/disk/index.md
+          - title: Disk management
+            location: stacks/disk/index.md
 #          - title: Location services
 #            location: stacks/location/docs/index.md
 #          - title: Media playback & recording
 #            location: stacks/media/docs/index.md
-#          - title: Network management & services
-#            location: stacks/network/index.md
+          - title: Network management & services
+            location: stacks/network/index.md
 #          - title: Power management
 #            location: stacks/power/docs/index.md
-#          - title: TPM
-#            location: stacks/tpm/index.md
+          - title: TPM
+            location: stacks/tpm/index.md
+
+  - title: Go to production
+
+    children:
+      - title: Introduction
+        location: guides/go-to-production/intro.md
+
+      - title: Simple - Ubuntu store
+        location: guides/go-to-production/simple.md
+
+      - title: Advanced - Branded store
+        location: guides/go-to-production/advanced.md
 
   - title: Reference
 
@@ -113,7 +113,6 @@ navigation:
       - title: Ubuntu Core Configuration
         location: reference/core-configuration.md
 
-
   - title: Build snaps
     location: http://snapcraft.io