style(images): add image best practices to styleguide doc

public/docs/_includes/styleguide/_images.jade

@@ -0,0 +1,10 @@
+#sg-images.showcase.shadow-1
+  header.showcase-header
+    h2 Images
+    p.
+      To maintain visual consistency across documentation chapters, please follow the best
+      practices for authors outlined in the <a href="/docs/ImageGuide.pdf" target="_blank">Image
+      Guide</a>.
+    p.
+     The browser background template used for outlining screenshots is <a
+     href="/resources/images/backgrounds/browser-background-template.png" target="_blank">here</a>.

public/docs/_includes/styleguide/_styleguide.jade

@@ -6,5 +6,6 @@
     != partial("_callouts")
     != partial("_tables")
     != partial("_aside")
+    != partial("_images")
 
 //!= partial("_jump-nav")