Extend deprecation/removal note that collections can be installed manually after removal

[felixfontein]

Similar to ansible-community/antsibull-build#647.

The output looks like this:

.. note::
    The google.cloud collection will be removed from Ansible 12 due to violations of the Ansible inclusion requirements.
    The collection has \ `unresolved sanity test failures <https://github.com/ansible-collections/google.cloud/issues/613>`__.
    Once removed, you can still install the collection manually with :literal:`ansible-galaxy collection install google.cloud`.
    See the \ `discussion thread <https://forum.ansible.com/t/8609>`__ for more information.

(The line "Once removed, ..." is new.)

[felixfontein]

@samccann thanks for reviewing this!