[FIX] website: use correct z-index for right panel top actions buttons

Since [1], the editor panel is now on the right. The top actions z-index
was not adapted and was now too high and going above modals. It worked
before because the zindex was confined into the topbar stacking context,
which does not exist anymore.

[1]: https://github.com/odoo/odoo/pull/57975



closes odoo/odoo#58237

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