Added the .navbar-expand class to the docs-nav navbar

Created by: gijsbotje

fixes #22976 (closed)

_includes/docs-navbar.html

-<header class="navbar navbar-dark flex-column flex-md-row bd-navbar">
+<header class="navbar navbar-expand navbar-dark flex-column flex-md-row bd-navbar">
   <a class="navbar-brand mr-0 mr-md-2" href="{{ site.baseurl }}/" aria-label="Bootstrap">
     {% include icons/bootstrap.svg width="36" height="36" class="d-block" %}
   </a>