docs: Fix copying code blocks (#11617)

This PR fixes copying code blocks in the docs.

The problem was that some of the elements we removed from the base
mdBook template were causing errors in the script, which prevented the
right event listeners from being registered for the copy button.

To remedy this, the elements have been restored, but are using `display:
none` so that they don't appear in the UI.

Resolves #11592.

Release Notes:

- N/A

docs/theme/index.hbs

@@ -91,6 +91,9 @@
             <div class="sidebar-scrollbox">
                 {{#toc}}{{/toc}}
             </div>
+            <div style="display: none;" id="sidebar-resize-handle" class="sidebar-resize-handle">
+                <div class="sidebar-resize-indicator"></div>
+            </div>
         </nav>
 
         <!-- Track and set sidebar scroll position -->
@@ -125,6 +128,16 @@
                         <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
                             <i class="fa fa-bars"></i>
                         </label>
+                        <button style="display: none;" id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
+                            <i class="fa fa-paint-brush"></i>
+                        </button>
+                        <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
+                            <li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
+                        </ul>
                         {{#if search_enabled}}
                         <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
                             <i class="fa fa-search"></i>