[FIX] website: allow elements to overflow fixed parallax

Parallax elements are overflow:hidden so that their moving background
does not overflow. This introduces a limitation: no dropdown can
overflow the snippet either.

While a better solution should be found in master, this allows overflow
for the "fixed" parallax which does not use an overflowing background.

opw-2091324

closes odoo/odoo#41492

X-original-commit: 72d3b016
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>