Add IT locale

[stefanodavid]

This PR adds Italian localisations to the PyData Sphinx Theme.

[drammock]

wonderful, thanks for the contribution! @12rambau is the localization expert around here, but I think I know enough to parrot him here and say "this needs to be done through transifex, because that way (here's where my knowledge is fuzzy) something important happens that won't happen automatically if we just add the .po file in a pull request

the relevant team is here: https://explore.transifex.com/12rambau/pydata-sphinx-theme/

[12rambau]

Hi @stefanodavid and thanks for offering your help on the translation effort.

To fasten the process I just added italian to the list of languages this very morning so you simply have to request access to team. As mentioned by @drammock the headers of the .po files are tricky and they should be edited upon every modification which is automatically managed by transifex.

happy to hear from you there: https://app.transifex.com/12rambau/pydata-sphinx-theme/dashboard/