[IMP] website: add s_alert snippet default layout options
[IMP] website: add s_alert snippet default layout options

Part of Inner Snippets review task. `s_alert` should have a list of preset
layout.

Part of https://github.com/odoo/odoo/pull/42937
task-2088157
task-2152191
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo