Change develop in nav bar to contribute

Way back when, this section of the docs was named contribute. We changed it to develop at some point but that feels too narrow so this restores the name of that section to contribute

Author: story645

mpl_sphinx_theme/mpl_nav_bar.html

@@ -15,7 +15,7 @@
     <a class="reference internal nav-link" href="{{ mpl_path('users/index') }}">User guide</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="{{ mpl_path('devel/index') }}">Develop</a>
+    <a class="reference internal nav-link" href="{{ mpl_path('devel/index') }}">Contribute</a>
   </li>
   <li class="nav-item">
     <a class="reference internal nav-link" href="{{ mpl_path('users/release_notes') }}">Releases</a>