Make non-native site links server-absolute

... instead of globally absolute with an explicit server name. This
allows the docs to be served from a different server (e.g., on
localhost), without links leaking out to the real matplotlib.org.

mpl_sphinx_theme/mpl_nav_bar.html

@@ -25,25 +25,25 @@
 {% else %}
 <ul id="navbar-main-elements" class="navbar-nav">
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/plot_types/index">Plot types</a>
+    <a class="reference internal nav-link" href="/stable/plot_types/index">Plot types</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/gallery/index">Examples</a>
+    <a class="reference internal nav-link" href="/stable/gallery/index">Examples</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/tutorials/index">Tutorials</a>
+    <a class="reference internal nav-link" href="/stable/tutorials/index">Tutorials</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/api/index">Reference</a>
+    <a class="reference internal nav-link" href="/stable/api/index">Reference</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/users/index">Usage guide</a>
+    <a class="reference internal nav-link" href="/stable/users/index">Usage guide</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/devel/index">Develop</a>
+    <a class="reference internal nav-link" href="/stable/devel/index">Develop</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="https://matplotlib.org/stable/users/release_notes">Release notes</a>
+    <a class="reference internal nav-link" href="/stable/users/release_notes">Release notes</a>
   </li>
 </ul>
-{% endif %}
+{% endif %}