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/_includes/section.html

@@ -0,0 +1,17 @@
+
+<div class="section {{ sectionClass }} vertical-align-middle">
+  <div class="wrapper">
+    <div class="description mbl">
+      <h2 class="section-title">{{ sectionTitle }}</h2>
+      {{ sectionContent }}
+    </div>
+    {% if sectionIcons %}
+      {% assign sectionIconsAsArray = sectionIcons | split:'~~' %}
+      <div class="icons">
+      {% for icon in sectionIconsAsArray %}
+        <img src="{{ site.github.url }}/images/{{ icon }}"/>
+      {% endfor %}
+      </div>
+    {% endif %}
+  </div>
+</div>