doc: point Documentation link at index page

Rather than hard-coding the spdk.io/doc link, change the Doxygen header
to output a link to index.html (the main Doxygen page) so that it will
always point within the current doc tree.

This is important for archived release versions of the documentation,
since their documentation link should point at their own main page, not
the current master doc.

Change-Id: I788eee995543e425c24ff787f3f13956c5b813b7
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

doc/header.html

@@ -51,7 +51,7 @@
               </li>
 
               <li class="active" role="presentation">
-                <a href="http://www.spdk.io/doc/">
+                <a href="index.html">
                   <i class="glyphicon glyphicon-book"></i>
                   <span class="box-name">documentation</span>
                 </a>