[FIX] website: disable scrolling effects for sidebar headers

- Edit the website
- Select the header and set the scrolling effect to fixed
- Set header template as 'sidebar'
- Save
- Scroll -> The website does not properly scroll.

This is now fixed.

closes odoo/odoo#69219

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