[FIX] website: restore background sizing of fixed parallax backgrounds

Fixed backgrounds are disabled when using a mobile/tablet device,
which is not a big deal but, on some of them (iOS...), defining the
background as fixed breaks the background-size/position props.
So we enable this only for >= XL devices.

We may want to find a better alternative for future versions.

opw-2055073

closes odoo/odoo#36187

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>