doc: add missing ARIA label for button

The button for toggling light mode and dark mode has no text display.
Screen readers will read it as simply "button", making it not useful.
Add an aria-label attribute so it gets a better description.

doc/template.html

@@ -25,7 +25,7 @@
       <header>
         <div class="header-container">
           <h1>Node.js __VERSION__ Documentation</h1>
-          <button class="theme-toggle-btn" id="theme-toggle-btn" hidden>
+          <button class="theme-toggle-btn" id="theme-toggle-btn" aria-label="Toggle dark mode/light mode" hidden>
             <svg xmlns="http://www.w3.org/2000/svg" class="icon dark-icon" height="24" width="24">
               <path fill="none" d="M0 0h24v24H0z" />
               <path d="M11.1 12.08c-2.33-4.51-.5-8.48.53-10.07C6.27 2.2 1.98 6.59 1.98 12c0 .14.02.28.02.42.62-.27 1.29-.42 2-.42 1.66 0 3.18.83 4.1 2.15A4.01 4.01 0 0111 18c0 1.52-.87 2.83-2.12 3.51.98.32 2.03.5 3.11.5 3.5 0 6.58-1.8 8.37-4.52-2.36.23-6.98-.97-9.26-5.41z"/>