[IMP] website: review snippet parallax
[IMP] website: review snippet parallax

Make the parallax snippet empty (remove the title). Remove the padding.
Add style to the building block with a message to help user to drag
stuff into the snippet.

Part of https://github.com/odoo/odoo/pull/42937
task-2088157
task-2152185
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo