Add letter-specific formatting advice

Letters can’t have callouts.

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

@@ -0,0 +1,19 @@
+<h2 class="heading-medium">Formatting</h2>
+<p>
+  To put a title in your template, use a hash:
+</p>
+<div class="panel panel-border-wide">
+  <p>
+    # This is a title
+  </p>
+</div>
+<p>
+  To make bullet points, use asterisks:
+</p>
+<div class="panel panel-border-wide">
+  <p>
+    * point 1<br/>
+    * point 2<br/>
+    * point 3<br/>
+  </p>
+</div>