Add rules button to join page

frontend/src/pages/Join.svelte

@@ -60,4 +60,8 @@
             </button>
         </form>
     </div>
+
+    <div class="welcome-links">
+        <a href="/rules" target="_blank" rel="noopener" class="btn btn-secondary">📖 Read the Rules</a>
+    </div>
 </div>