build(aio): move autolink-headings to post-processing (#16336)

The autolinking is now done on the `renderedContent` which means it also
captures and autolinks headings that were generated outside of markdown.

PR Close #16336

aio/tools/transforms/angular-base-package/post-processors/autolink-headings.js

@@ -0,0 +1,22 @@
+const slug = require('rehype-slug');
+const link = require('rehype-autolink-headings');
+
+/**
+ * Get remark to inject anchors into headings
+ */
+module.exports = [
+  slug,
+  [link, {
+    properties: {
+      title: 'Link to this heading',
+      className: ['header-link'],
+      'aria-hidden': 'true'
+    },
+    content: {
+      type: 'element',
+      tagName: 'i',
+      properties: {className: ['material-icons']},
+      children: [{ type: 'text', value: 'link' }]
+    }
+  }]
+];