Update product page to talk about letters

Letters is now a mature enough feature that we should:
- be raising awareness amongst our users that it’s a thing we offer
- not have letters be a surprise to anyone creating a Notify account for
  the first time

Shouldn’t be merged until:
- [ ] https://github.com/alphagov/notifications-api/pull/1600

app/assets/stylesheets/_grids.scss

@@ -95,6 +95,10 @@
   text-align: right;
 }
 
+.align-with-big-number-hint {
+  margin-top: $gutter / 0.6;
+}
+
 .global-cookie-message {
   p {
     @extend %site-width-container;

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

@@ -16,7 +16,7 @@
         background-image: file-url('product/proposition-illustration.png');
         background-size: 320px;
         background-repeat: no-repeat;
-        background-position: right -6px top 85px;
+        background-position: right -6px top 100px;
 
         @include ie-lte(8) {
           background-image: none;