docs: a little more marketing

docs/_templates/github.html

 <p>
 <br>
+<a class="github-button" href="https://github.com/dw/mitogen" data-size="large" data-show-count="true" aria-label="Star dw/mitogen on GitHub">Star</a>
+</p>
+
+<p>
 <a href="https://github.com/dw/mitogen/">GitHub Repository</a>
 </p>

docs/_templates/layout.html

@@ -17,4 +17,6 @@
       })();
     </script>
     <noscript><p><img src="http://37.187.23.96/tr/ep?idsite=6" style="border:0" alt=""></p></noscript>
+
+    <script async defer src="https://buttons.github.io/buttons.js"></script>
 {% endblock %}