Add a grid of features to the home page

This is the first step towards a fully-fledged ‘product’ page.

The needs for the top, blue section of the page are:
- what is Notify?
- can I use it?
- can I test it out?
- how do I create a log in

The needs for the next section of this page (which is the only one added
by this commit) are:
- how will Notify help me work?
- will it work with my service?

This commit shows 4 features of Notify on the home page which address
those needs. They are illustrated because:

1. We want to catch people’s attention – users are reluctant to scroll
   on this page because they just want to click the ‘create account
   button’. But we hypothesize that they will get on better
   with Notify if they look at some of this stuff first.

2. The concepts that they’re talking about are hard to explain with just
   words because they’re quite abstract. The illustrations help us be
   more specific.

3. Feedback we got from user research was that the product page didn’t
   give users any sense of what it was like to actually use Notify.

app/assets/stylesheets/views/product-page.scss

@@ -60,8 +60,19 @@
   }
 
   &-section {
+
     @extend %site-width-container;
-    margin-bottom: $gutter * 1.5;
+
+    h2 {
+      @include bold-24;
+      margin: $gutter-half 0 10px 0;
+    }
+
+    img {
+      max-width: 100%;
+      margin: 0 0 $gutter * 1.5 0;
+    }
+
   }
 
 }