Framateam: write a very minimal documentation in English

gasche requested to merge gasche/docs:mattermost-en into master

Currently the "About > Documentation" entry in the top Framasoft bar from Framateam brings you to a dead link 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.

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