Update links

support/rtd/index.rst

 If you are not redirected automatically, follow the
-`link to the C++ Format documentation <http://cppformat.github.io/latest/>`_.
+`link to the fmt documentation <http://fmtlib.net/latest/>`_.

support/rtd/theme/layout.html

@@ -2,15 +2,15 @@
 
 {% block extrahead %}
 <meta charset="UTF-8">
-<meta http-equiv="refresh" content="1;url=http://cppformat.github.io/latest/">
+<meta http-equiv="refresh" content="1;url=http://fmtlib.net/latest/">
 <script type="text/javascript">
-    window.location.href = "http://cppformat.github.io/latest/"
+    window.location.href = "http://fmtlib.net/latest/"
 </script>
 <title>Page Redirection</title>
 {% endblock %}
 
 {% block document %}
-If you are not redirected automatically, follow the <a href='http://cppformat.github.io/latest/'>link to the C++ Format documentation</a>.
+If you are not redirected automatically, follow the <a href='http://fmtlib.net/latest/'>link to the fmt documentation</a>.
 {% endblock %}
 
 {% block footer %}