source/_templates/navbar-version.html

{% if pagename not in ["index", "404"] and theme_logo.get("version_link") %}
{% if hasdoc(theme_logo.get("version_link")) %}
  {% set href = pathto(theme_logo.get("version_link")) %} {# internal page #}
{% else %}
  {% set href = theme_logo.get("version_link") %} {# external url #}
{% endif %}
<a class="navbar-brand version" href="{{ href }}">
  <p>v{{ version }}</p>
</a>
{% endif %}