Add layout.html template to set hreflang attributes

Tentative fix for search engines pointing English-seeking users
to localized docs.

_templates/layout.html

@@ -0,0 +1,14 @@
+{% extends "!layout.html" %}
+{% block linktags %}
+    <link rel="alternate" hreflang="en" href="http://docs.godotengine.org/en/" />
+    <link rel="alternate" hreflang="de" href="http://docs.godotengine.org/de/" />
+    <link rel="alternate" hreflang="es" href="http://docs.godotengine.org/es/" />
+    <link rel="alternate" hreflang="fr" href="http://docs.godotengine.org/fr/" />
+    <link rel="alternate" hreflang="ko" href="http://docs.godotengine.org/ko/" />
+    <link rel="alternate" hreflang="pl" href="http://docs.godotengine.org/pl/" />
+    <link rel="alternate" hreflang="pt-br" href="http://docs.godotengine.org/pt-br/" />
+    <link rel="alternate" hreflang="uk" href="http://docs.godotengine.org/uk/" />
+    <link rel="alternate" hreflang="zh-cn" href="http://docs.godotengine.org/zh-cn/" />
+    <link rel="alternate" hreflang="x-default" href="http://docs.godotengine.org/" />    
+    {{ super() }}
+{% endblock %}