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/package.json

@@ -83,10 +83,10 @@
     "lodash": "^4.17.4",
     "protractor": "~5.1.0",
     "rehype": "^4.0.0",
+    "rehype-autolink-headings": "^2.0.0",
+    "rehype-slug": "^2.0.0",
     "remark": "^7.0.0",
-    "remark-autolink-headings": "^4.0.0",
     "remark-html": "^6.0.0",
-    "remark-slug": "^4.2.2",
     "rho": "https://github.com/petebacondarwin/rho#master",
     "rimraf": "^2.6.1",
     "shelljs": "^0.7.7",