Update navigation links to use full URLs

Author: mchav

_includes/header.html

@@ -3,8 +3,8 @@ <h1 id="logo"><a href="index.html">data<span>Haskell</span></a></h1>
   <nav id="nav">
     <!-- Header -->
     <ul>
-      <li class="current"><a href="index.html">Home</a></li>
-      <li class=""><a href="blog.html">Blog</a></li>
+      <li class="current"><a href="https://datahaskell.org/index.html">Home</a></li>
+      <li class=""><a href="https://datahaskell.org/blog.html">Blog</a></li>
       <li class=""><a href="https://datahaskell.org/docs">Docs</a></li>
       <li class=""><a href="https://www.datahaskell.org/docs/community/roadmap.html">Roadmap</a></li>
       <li class=""><a href="https://github.com/DataHaskell">Github</a></li>