tools: add navigation ARIA landmark to generated API ToC

As an accessibility improvement, specify the navigation landmark for
the element of our docs that contains the table of contents generated
for the specific API page.

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>

tools/doc/html.mjs

@@ -467,7 +467,7 @@ export function buildToc({ filename, apilinks }) {
         .use(htmlStringify)
         .processSync(toc).toString();
 
-      file.toc = `<details id="toc" open><summary>Table of contents</summary>${inner}</details>`;
+      file.toc = `<details role="navigation" id="toc" open><summary>Table of contents</summary>${inner}</details>`;
       file.tocPicker = `<div class="toc">${inner}</div>`;
     } else {
       file.toc = file.tocPicker = '<!-- TOC -->';