DOC: try using non-breaking spaces in nav bar

This may help the responsive layout to avoid things like
matplotlib/matplotlib#28832

Author: tacaswell

mpl_sphinx_theme/mpl_nav_bar.html

@@ -1,9 +1,9 @@
 <ul id="navbar-main-elements" class="navbar-nav">
   <li class="nav-item">
-    <a class="reference internal nav-link" href="{{ mpl_path('plot_types/index') }}">Plot types</a>
+    <a class="reference internal nav-link" href="{{ mpl_path('plot_types/index') }}">Plot&nbsp;types</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="{{ mpl_path('users/index') }}">User guide</a>
+    <a class="reference internal nav-link" href="{{ mpl_path('users/index') }}">User&nbsp;guide</a>
   </li>
   <li class="nav-item">
     <a class="reference internal nav-link" href="{{ mpl_path('tutorials/index') }}">Tutorials</a>