fix(aio): show embedded ToC (#23944)

On narrow screens (where there is not enough room on the right to show
the floating ToC), an embedded ToC is shown (via an `<aio-toc embedded>`
element in the document). Since ToC was not a custom element, the
component was not instantiated for the embedded element.

This commit fixes it by making `aio-toc` a custom element and loading it
manually for the floating ToC (if necessary).

PR Close #23944

aio/scripts/_payload-limits.json

@@ -2,8 +2,8 @@
   "aio": {
     "master": {
       "uncompressed": {
-        "runtime": 2712,
-        "main": 479729,
+        "runtime": 2768,
+        "main": 475857,
         "polyfills": 38453,
         "prettify": 14913
       }