Set up gh-pages branch

The index.html will redirect people to the latest documentation
directory (develop), and the .nojekyll file will force github
to serve index.html files on a folder path (the user would be
served synapse/develop/index.html if they navigated to
synapse/develop).

index.html

@@ -0,0 +1,6 @@
+<title> Synapse Documentation </title>
+<meta http-equiv="Refresh" content="0; url='https://matrix-org.github.io/synapse/develop/index.html'" />
+<p>
+    Redirecting you to the latest documentation.
+        If you are not redirected, please <a href="https://matrix-org.github.io/synapse/develop/index.html">click here</a>.
+</p>