Update FR userdocs

This is the first translate of PeerTube User Docs. I updated sidebar file like this :

• Utiliser PeerTube
• Administrer PeerTube
• Installer PeerTube
• Maintain PeerTube (sysadmin)
• Contribuer à PeerTube
• PeerTube API

oups, i forgot to translate Maintain :)

I think is better for users, admin, dev and contributors. Hope it's ok because i can't test links...