Move documentation from gh-pages branch to 'docs' folder

To prepare the migration to 3.0, we need to change the location where the documentation is generated from.
Having a gh-pages branch makes it hard to synchronize the code and the docs. Having a "docs" folder in the repo itself simplifies this.

docs/_layouts/page.html

@@ -0,0 +1,14 @@
+---
+layout: default
+---
+<article class="post">
+  <div class="wrapper">
+  <header class="post-header">
+    <h1 class="post-title">{{ page.title }}</h1>
+  </header>
+  <div class="post-content">    
+      {{ content }}    
+  </div>
+  </div>
+
+</article>