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
6e05ae02a2