fix(aio): fix scrolling to top (#17102)

Previously, the `#top-of-page` element (used when scrolling to top) was placed
inside the content section (which at the time had zero top margin and padding).
Furthermore, there was a top offset applied when scrolling that took the static
top bar's height into account. Since now the top bar is not static any more and
the content section has a non-zero top padding, scrolling to top does not work
as expected.

This commit fixes this by:
- Moving the `#top-of-page` element to the top of the `aio-shell`.
- Stop accounting for the top bar's top.

Fixes #17006