docs: show nav menu for mobile (#2371)

docs/overrides/main.html

@@ -38,6 +38,13 @@
       display: none;
     }
 
+    /* Show sidebar on mobile */
+    @media screen and (max-width: 768px) {
+      .md-sidebar--primary {
+        display: block;
+      }
+    }
+
     .md-typeset a:hover {
       color: #1565C0;
     }