Add Namespace link in panel

The `list.svg` file is from [Feather icons][feather] v4.29.0, and is
[licensed under the MIT license][license].

[feather]: https://feathericons.com/
[license]: https://github.com/feathericons/feather/blob/v4.29.0/LICENSE

lib/rdoc/generator/template/rails/_context.rhtml

@@ -156,7 +156,7 @@
 
   <% unless @context.classes_and_modules.empty? %>
     <!-- Namespace -->
-    <div class="content__divider">Namespace</div>
+    <div id="namespace" class="content__divider">Namespace</div>
     <ul>
       <% (@context.modules.sort + @context.classes.sort).each do |mod| %>
         <li>

lib/rdoc/generator/template/rails/_module_nav.rhtml

@@ -1,3 +1,6 @@
+<% unless @context.classes_and_modules.empty? %>
+  <a href="#namespace" class="namespace-link"><%= short_name(@context) %> namespace</a>
+<% end %>
 <%= button_to_search @context, display_name: short_name(@context) %>
 
 <% if outline = outline(@context) %>

lib/rdoc/generator/template/rails/resources/css/main.css

@@ -74,14 +74,7 @@ a:has(> code):not(:has(> :not(code))) {
   text-align: left;
   font-family: inherit;
   font-size: 1em;
-
-  background: url('../i/filter.svg') no-repeat;
-  background-position-y: 1px;
-  background-size: 1.1em;
-  padding: 0 0 0 1.3em;
   color: var(--link-color);
-
-  word-break: break-word;
 }
 
 @media (hover: hover) {
@@ -461,6 +454,28 @@ html {
 }
 
 
+.panel__nav :is(.namespace-link, .query-button) {
+  display: block;
+  word-break: break-word;
+}
+
+.panel__nav .namespace-link {
+  background: url('../i/list.svg') no-repeat;
+  background-position-y: 1px;
+  background-size: 1.1em;
+  padding: 0 0 0 1.3em;
+
+  text-decoration: none;
+}
+
+.panel__nav .query-button {
+  background: url('../i/filter.svg') no-repeat;
+  background-position-y: 1px;
+  background-size: 1.1em;
+  padding: 0 0 0 1.3em;
+}
+
+
 /*
  * Navigation panel on desktop
  */