Docs: enhance 'On this page' ToC accessibility (#41483)

Co-authored-by: Julien Déramond <[email protected]>

Author: louismaximepiton

site/src/layouts/DocsLayout.astro

@@ -97,10 +97,10 @@ if (frontmatter.toc) {
                 <use xlink:href="#chevron-expand" />
               </svg>
             </button>
-            <strong class="d-none d-md-block h6 my-2 ms-3">On this page</strong>
+            <strong class="d-none d-md-block h6 my-2 ms-3" id="docs-tocs">On this page</strong>
             <hr class="d-none d-md-block my-2 ms-3" />
             <div class="collapse bd-toc-collapse" id="tocContents">
-              <nav id="TableOfContents">
+              <nav id="TableOfContents" aria-labelledby="docs-tocs">
                 <TableOfContents headings={headings} />
               </nav>
             </div>