docs: it_IT: address invalid reference warnings

docs: it_IT: address invalid reference warnings

Documentation generation warns:

  it_IT/kernel-hacking/hacking.rst:
    WARNING: unknown document: ../core-api/symbol/namespaces

  it_IT/process/5.Posting.rst:
    WARNING: undefined label: it_email_clients

  it_IT/process/submitting-patches.rst:
    WARNING: undefined label: it_email_clients

  it_IT/process/howto.rst:
     WARNING: undefined label: it_managementstyle

Refer to English documentation, as Italian translation does not exist,
and add labels for Italian process documents to resolve label references.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200531185618.7099-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>