doc:it_IT: add symbol-namespace translation

- add complete translation of symbol-namespaces.rst
- fix references to this page within the italian translation
- add document to main indexes

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Link: https://lore.kernel.org/r/20200614201053.59502-1-federico.vaga@vaga.pv.it


Signed-off-by: Jonathan Corbet <corbet@lwn.net>