[analyzer] Add the Checker Developer Manual to the menu.

It's still under construction but has enough info to be useful.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143912 91177308-0d34-0410-b5e6-96231b3b80d8

www/analyzer/menu.html.incl

@@ -25,6 +25,7 @@
 <li>
   Development
   <ul>
+    <li><a href="/checker_dev_manual.html">Checker Developer Manual</a></li>
     <li><a href="/dev_cxx.html">Analysis support for C++</a></li>
   </ul>
 </li>