MINOR: Fix document header/footer links

Based on:  https://github.com/apache/kafka-site/pull/27

I recommend also merging into 10.1.0.0 branch to avoid mishaps when re-publishing docs.

Author: Gwen Shapira <cshapi@gmail.com>

Reviewers: Guozhang Wang <wangguoz@gmail.com>

Closes #2077 from gwenshap/protocol_doc_fix

docs/migration.html

@@ -15,7 +15,7 @@
  limitations under the License.
 -->
 
-<!--#include virtual="../includes/header.html" -->
+<!--#include virtual="../includes/_header.html" -->
 <h2><a id="migration" href="#migration">Migrating from 0.7.x to 0.8</a></h2>
 
 0.8 is our first (and hopefully last) release with a non-backwards-compatible wire protocol, ZooKeeper     layout, and on-disk data format. This was a chance for us to clean up a lot of cruft and start fresh. This means performing a no-downtime upgrade is more painful than normal&mdash;you cannot just swap in the new code in-place.
@@ -31,4 +31,4 @@
     <li>Drink.
 </ol>
 
-<!--#include virtual="../includes/footer.html" -->
+<!--#include virtual="../includes/_footer.html" -->

docs/protocol.html

@@ -15,10 +15,10 @@
  limitations under the License.
 -->
 
-<!--#include virtual="../includes/header.html" -->
-<!--#include virtual="../includes/top.html" -->
+<!--#include virtual="../includes/_header.html" -->
+<!--#include virtual="../includes/_top.html" -->
 <div class="content">
-    <!--#include virtual="../includes/nav.html" -->
+    <!--#include virtual="../includes/_nav.html" -->
     <div class="right">
         <h1>Kafka protocol guide</h1>
 
@@ -228,4 +228,4 @@ Size => int32
         $(function() { $('.b-nav__project').addClass('selected'); });
     </script>
 
-<!--#include virtual="../includes/footer.html" -->
+<!--#include virtual="../includes/_footer.html" -->