Add Open Graph tags for better link preview on social media

This is provided by the sphinxext-opengraph extension.

This also adds a theme color property in the HTML layout file.

_templates/layout.html

@@ -1,5 +1,6 @@
 {% extends "!layout.html" -%}
 {% block linktags -%}
+  <meta name="theme-color" content="#3d8fcc">
   {% if godot_inject_language_links -%}
   {% for alternate_lang in godot_docs_supported_languages -%}
   {# Convert to ISO 639-1 format, e.g. zh_CN -> zh-cn -#}