Update styles to enable printing docs (vuejs#1425) (vuejs#1426)

The sidebar, header, footer and Advertisement elements are now hidden
when users print docs from the website.

Author: NiloCK

themes/vue/source/css/_ad.styl

@@ -64,3 +64,7 @@
       margin-left 6px
   .default-ad
     display none
+
+@media print
+  #ad
+    display: none

themes/vue/source/css/_header.styl

@@ -127,3 +127,7 @@ body.docs
     left: 50%
     margin-left: -15px
     background-size: 30px
+
+@media print
+  #header
+    display: none

themes/vue/source/css/_sidebar.styl

@@ -89,3 +89,7 @@
     &.open
       -webkit-transform: translate(0, 0)
       transform: translate(0, 0)
+
+@media print
+  .sidebar
+    display: none

themes/vue/source/css/page.styl

@@ -240,3 +240,7 @@
       margin-left: 0
   iframe
     margin: 0 !important
+
+@media print
+  .footer
+    display: none