[FIX] website: properly go to fixed anchors

Since the header effects are now done without duplicating the header
into a fixed element, link anchors towards it were not working anymore
as they could be redirected to the fixed-positioned header. This commit
solves the problem "generically" for all fixed elements (I am not sure
any link towards a fixed element except the header makes sense though).

closes odoo/odoo#53156

X-original-commit: 1a2827f80bd7a66d4190977b22d4bc5846737b08
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>