Fix header links to stable docs from subdirectories

Author: speth

source/_templates/initial-sections.html

@@ -1,25 +1,25 @@
 <nav class="navbar-nav">
   <ul class="bd-navbar-elements navbar-nav">
     <li class="nav-item">
-      <a class="nav-link nav-internal" href="stable/install/index.html">Install</a>
+      <a class="nav-link nav-internal" href="/stable/install/index.html">Install</a>
     </li>
 
     <li class="nav-item">
-      <a class="nav-link nav-internal" href="stable/userguide/index.html">User Guide</a>
+      <a class="nav-link nav-internal" href="/stable/userguide/index.html">User Guide</a>
     </li>
 
 
     <li class="nav-item">
-      <a class="nav-link nav-internal" href="stable/examples/index.html">Examples</a>
+      <a class="nav-link nav-internal" href="/stable/examples/index.html">Examples</a>
     </li>
 
 
     <li class="nav-item">
-      <a class="nav-link nav-internal" href="stable/reference/index.html">Reference</a>
+      <a class="nav-link nav-internal" href="/stable/reference/index.html">Reference</a>
     </li>
 
     <li class="nav-item">
-      <a class="nav-link nav-internal" href="stable/develop/index.html">Develop</a>
+      <a class="nav-link nav-internal" href="/stable/develop/index.html">Develop</a>
     </li>
 
   </ul>