[Docs] Force using current protocol scheme for resources and links

If the docs is accessed with https, change all links to be https as well.

docs/_includes/head.html

@@ -6,7 +6,7 @@
   <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 }}">
+  <link rel="stylesheet" href="{{ "/css/main.css" | prepend: site_url }}">
   {% if site.algolia.enabled %}
   <link rel="stylesheet" href="//cdn.jsdelivr.net/docsearch.js/1/docsearch.min.css" />
   {% endif %}