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/base.html

@@ -0,0 +1,9 @@
+<!-- http://ricostacruz.com/til/relative-paths-in-jekyll.html -->
+<!-- _includes/base.html -->
+
+{% assign base = '' %}
+{% assign depth = page.url | split: '/' | size | minus: 1 %}
+{% if    depth == 1 %}{% assign base = '.' %}
+{% elsif depth == 2 %}{% assign base = '..' %}
+{% elsif depth == 3 %}{% assign base = '../..' %}
+{% elsif depth == 4 %}{% assign base = '../../..' %}{% endif %}