doc: fix scrolling glitch causing search bar to be partially hidden

With the recent update to Sphinx RTD theme 3.0.0, a 19px-high element
that used to display the version number is not visible anymore, so the
JS code that hides the upper-left logo when scrolling down needs to be
adjusted to account for this change.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>