Use U+2630 as sidebar toggle icon

lib/rdoc/generator/template/darkfish/_sidebar_toggle.rhtml

@@ -1,26 +1,3 @@
 <button id="navigation-toggle" class="navigation-toggle" aria-label="Toggle sidebar" aria-expanded="true" aria-controls="navigation">
-  <!--
-    Copyright (c) 2021 Luca Burgio
-
-    Permission is hereby granted, free of charge, to any person obtaining a copy
-    of this software and associated documentation files (the "Software"), to deal
-    in the Software without restriction, including without limitation the rights
-    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-    copies of the Software, and to permit persons to whom the Software is
-    furnished to do so, subject to the following conditions:
-
-    The above copyright notice and this permission notice shall be included in all
-    copies or substantial portions of the Software.
-
-    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-    SOFTWARE.
-  -->
-  <svg width="20px" height="24px" stroke-width="1.5" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" color="#000000">
-    <path d="M3 5h18M3 12h18M3 19h18" stroke="#000000" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"></path>
-  </svg>
+  <span aria-hidden>☰</span>
 </button>

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -299,14 +299,13 @@ nav p {
 .navigation-toggle {
   position: fixed;
   left: 4px;
-  top: 6px;
   z-index: 100;
 
   background: transparent;
   border: none;
   cursor: pointer;
-  margin-left: 7px;
   padding: 4px;
+  font-size: 24px;
 }
 .navigation-toggle[aria-expanded="true"] {
   left: 260px;