Add guidance on how to use page breaks in letters

Useful generally, but especially since we’ve edited some team’s
templates to use this new feature, they could probably do with knowing
how it works.

app/templates/partials/templates/guidance-formatting-letters.html

@@ -17,3 +17,15 @@
     * point 3<br/>
   </p>
 </div>
+<p>
+  To insert a page break, use three asterisks:
+</p>
+<div class="panel panel-border-wide">
+  <p>
+    Content on page 1<br/>
+    <br/>
+    ***<br/>
+    <br/>
+    Content on page 2<br/>
+  </p>
+</div>