doc: add navigation ARIA landmark to doc ToC

As an accessibility improvement, specify the navigation landmark for
the column in our docs that contains the table of contents.

Ref: https://www.w3.org/WAI/ARIA/apg/practices/landmark-regions/
Ref: https://www.w3.org/WAI/WCAG21/Techniques/aria/ARIA20.html
PR-URL: https://github.com/nodejs/node/pull/49882
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Claudio Wunder <cwunder@gnome.org>
Reviewed-By: LiviaMedeiros <livia@cirno.name>

doc/template.html

@@ -14,7 +14,7 @@
 </head>
 <body class="alt apidoc" id="api-section-__FILENAME__">
   <div id="content" class="clearfix">
-    <div id="column2" class="interior">
+    <div role="navigation" id="column2" class="interior">
       <div id="intro" class="interior">
         <a href="/" title="Go back to the home page">
           Node.js