Update permalinks for accessibility (#36714)

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

lib/render-content/plugins/permalinks.js

@@ -0,0 +1,23 @@
+import { visit } from 'unist-util-visit'
+import { h } from 'hastscript'
+
+const matcher = (node) =>
+  node.type === 'element' &&
+  ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'].includes(node.tagName) &&
+  node.properties?.id
+
+export default function permalinks() {
+  return (tree) => {
+    visit(tree, matcher, (node) => {
+      const { id } = node.properties
+      const text = node.children
+      node.properties.tabIndex = -1
+      node.children = [
+        h('a', { class: 'permalink', href: `#${id}` }, [
+          ...text,
+          h('span', { class: 'permalink-symbol', ariaHidden: 'true' }, [' #']),
+        ]),
+      ]
+    })
+  }
+}