[Notifier] Add Sipgate bridge

[sakul95]

Add a docs entry according to PR #sakul95:sipgate_notifier_transport

[OskarStark]

Please add

.. versionadded:: 7.2

    The ``Sipgate`` integration was introduced in Symfony 7.2.

below the versionadded:: 7.1

Thanks

[xabbuh]

Thank you @sakul95.