Draft : Add publiccode file for Italy's PublicCode catalog

This merge request aim to provide public code manifest that permit Okular to be included into the italian public code catalog as described by the project "Getting KDE project into Italy's PublicCode catalog" https://phabricator.kde.org/T14079

I used the information present in the desktop file and Okular website.

This manifest was validated against the official parser provided by the Italian government.

docker run -i italia/publiccode-parser-go /dev/stdin < publiccode.yml
validation ok