doc:it_IT: translation for doc-guide

This is the complete translation of 'doc-guide' in Italian.

Please note that code comments will stay in English because
thats the language that everyone should use to write comments.

External files (svg, dot) are taken from Documentation/doc-guide
instead of copying them into the Italian translation.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Alessia Mantegazza <amantegazza@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>