Release of G'MIC 3.0

@David_Tschumperle , hope you have good vacations.

Looking into the documentation, it would be very helpful for me to see or find easily new or changed (custom) commands, e.g. * new or changed command, † new or changed tutorial. Seemingly tutorial parts are isolated files, easily to recognize, changed or new custom commands are possibly recognized by diff in the update.gmic . I don’t know if the update.gmic could be stored in git for the differences?