doc: add main ARIA landmark to API docs

As an accessibility improvement, specify the main landmark for
the column in our docs that contains the main content.

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

doc/template.html

@@ -70,7 +70,7 @@ <h1>Node.js __VERSION__ documentation</h1>
 
       __TOC__
 
-      <div id="apicontent">
+      <div role="main" id="apicontent">
         __CONTENT__
         <!-- API END -->
       </div>

tools/doc/allhtml.mjs

@@ -27,7 +27,7 @@ for (const link of toc.match(/<a.*?>/g)) {
   const data = fs.readFileSync(new URL(`./${href}`, source), 'utf8');
 
   // Split the doc.
-  const match = /(<\/ul>\s*)?<\/\w+>\s*<\w+ id="apicontent">/.exec(data);
+  const match = /(<\/ul>\s*)?<\/\w+>\s*<\w+ role="main" id="apicontent">/.exec(data);
 
   // Get module name
   const moduleName = href.replace(/\.html$/, '');
@@ -89,7 +89,7 @@ all = all.slice(0, tocStart.index + tocStart[0].length) +
   all.slice(tocStart.index + tocStart[0].length);
 
 // Replace apicontent with the concatenated set of apicontents from each source.
-const apiStart = /<\w+ id="apicontent">\s*/.exec(all);
+const apiStart = /<\w+ role="main" id="apicontent">\s*/.exec(all);
 const apiEnd = all.lastIndexOf('<!-- API END -->');
 all = all.slice(0, apiStart.index + apiStart[0].length)
     .replace(