documize-community: 3.5.0 -> 3.5.1

https://github.com/documize/community/releases/tag/v3.5.1

This patch also switches to a `go.mod`-based build as upstream already
switched to it a while ago[1].

[1] https://github.com/documize/community/commit/b8268521372cdf8862af4f6924ec476b3e431690