docs/source/_templates/layout.html

{# Import the theme's layout. #}
{% extends "!layout.html" %}

{%- block footer %}
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-107500873-1"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-107500873-1');
</script>
{% endblock %}