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/algolia.html

@@ -0,0 +1,9 @@
+{% if site.algolia.enabled %}
+<script type="text/javascript" src="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.js"></script>
+<script type="text/javascript"> docsearch({
+    apiKey: '{{ site.algolia.apiKey }}',
+    indexName: '{{ site.algolia.indexName }}',
+    inputSelector: '#algolia-doc-search'
+});
+</script>
+{% endif %}