documentation/themes/beastie/i18n/it.toml: Various improvements

• Remove Oxford commas: they are not used in Italian.

• Translate GitHub and GitLab tradermarks lines, which have been added with commit 02a2c746a63591012e5a7a44f026c82c6ffc4a44.

Approved by: dbaio (co-mentor), carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D39801