chore(docs-app): fix vertical scrolling offset after recent re-design

Previously, the `yOffset` pointed to the `<header>` element, which had a height
of 0 (since all its children have fixed positions). This caused scrolled items
to have 0 vertical offset, essentially hiding the top part behind the static
`<nav>` items.
This commit fixes the vertical scrolling offset, by setting `yOffset` to the
last (lowest) `<nav>` item.

Closes #15945