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/icon-github.html

@@ -0,0 +1 @@
+<a href="https://github.com/{{ include.username }}"><span class="icon icon--github">{% include icon-github.svg %}</span><span class="username">{{ include.username }}</span></a>