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/head.html

@@ -0,0 +1,17 @@
+<head>
+  <meta charset="utf-8">
+  <meta http-equiv="X-UA-Compatible" content="IE=edge">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+
+  <title>{% if page.title %}{{ page.title | escape }}{% else %}{{ site.title | escape }}{% endif %}</title>
+  <meta name="description" content="{% if page.excerpt %}{{ page.excerpt | strip_html | strip_newlines | truncate: 160 }}{% else %}{{ site.description }}{% endif %}">
+
+  <link rel="stylesheet" href="{{ "/css/main.css" | prepend: site.github.url }}">
+  {% if site.algolia.enabled %}
+  <link rel="stylesheet" href="//cdn.jsdelivr.net/docsearch.js/1/docsearch.min.css" />
+  {% endif %}
+
+  {% if jekyll.environment == 'production' %}
+  {% include analytics.html %}
+  {% endif %}
+</head>