Framateam: write a very minimal documentation in English
Currently the "About > Documentation" entry in the top Framasoft bar from Framateam brings you to a dead link https://docs.framasoft.org/en/mattermost/ if the menu is in English version. The present commit populates the webpage at this URL by a minimal explanation that Framateam is powered by Mattermost, and a redirection to the upstream Mattermost documentation.