[FIX] website: boxed title style for splitting

When a title with boxed style (border arround title) is split (by doing
normal ENTER (and not shift ENTER which work as expected)) the title
being in inline-block for styling purpose, this give odd results (of
several title boxes on the same line).

With this commit style is changed so split title are on different lines.

opw-2070611
closes #37084

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>