Add a Random Module button (UniMath#1918)

This PR adds a button to the top menu bar which takes the user to a
random formalization module.

Not terribly discoverable, but I think that should be fixed by a
separate PR with a "how to use the website" guide which we're currently
missing.

<img width="541" height="121" alt="image"
src="https://github.com/user-attachments/assets/4a6b8acf-d34d-4445-8027-48b85f350d28"
/>

Author: VojtechStep

website/theme/index.hbs

@@ -166,6 +166,10 @@
                             <i class="fa fa-search"></i>
                         </button>
                         {{/if}}
+                        <button id="random-page" class="icon-button" type="button" title="Random Agda module"
+                            aria-label="Go to a random Agda module" aria-expanded="false">
+                            <i class="fa fa-random"></i>
+                        </button>
                     </div>
 
                     <h1 class="menu-title"><a href="HOME.html">{{ book_title }}</a></h1>
@@ -261,6 +265,25 @@
             {{/if}}
         </div>
 
+        <script>
+          document.querySelector('#random-page').addEventListener('click', () => {
+              // Skip meta pages, which all have names in capital letters
+              const metaRegex = /[A-Z_]+(\.lagda)?\.html/;
+              // Extract the module list from the sidebar, no point duplicating this data
+              const allModules = Array.prototype.filter.call(
+                  document.querySelectorAll('#sidebar .chapter-item > a'),
+                  // Filter based on the unresolved attribute (i.e. the relative path)
+                  link => {
+                      const href = link.attributes.href;
+                      // Filter out chevron links and meta pages
+                      return href && !metaRegex.test(href.value);
+                  }
+              );
+              const chosen = allModules[Math.floor(Math.random() * (allModules.length + 1))];
+              location.href = chosen.href;
+          });
+        </script>
+
         {{#if live_reload_endpoint}}
         <!-- Livereload script (if served using the cli tool) -->
         <script>