Add an id to all articles

Summary:
This should provide a better place for the search indexer to link to
Closes facebook#1123

Test Plan:
Loaded http://localhost:9811/rule/gen_aidl.html#content and saw that the
page was navigated to where I expected it to be.

Reviewed By: JoelMarcey, sdwilsh

fbshipit-source-id: 69d7f1a

docs/__common.soy

@@ -152,7 +152,7 @@ src="https://www.facebook.com/tr?id=1637165926500152&ev=PageView&noscript=1"
 	<section class='content'><div class='width'>
 
   {if $navid != 'home'}
-    <article>
+    <article id="content">
     <h1>{$title}{if $subtitle}:<div class="subtitle">{$subtitle}</div>{/if}</h1>
   {else}
     <landing-page>