doc: ownership verification tag

docs/_templates/layout.html

@@ -1,4 +1,5 @@
 {% extends "!layout.html" %}
 {% block extrahead %}
+<meta name="google-site-verification" content="0esgS4FkYROvPe-I9MYvC1tTYtfASPpqxBdpTHEBIcQ" />
 <script async defer src="https://buttons.github.io/buttons.js"></script>
 {% endblock %}