Writes the public Settings guide from Gamend.Settings.all/0.
mix gamend.settings.guide # write the guide
mix gamend.settings.guide --check # fail if it is out of date
mix gamend.settings.guide -o PATH # write somewhere elseSibling of mix gamend.settings.env_example, for the same reason: the
guides used to hand-list environment variables, and every rename left them
describing variables the server no longer read. A declared setting is now
documented for readers of the docs site the moment it exists.
Only for hosts that have a docs site. When priv/docs/60-operations
does not exist the task says so and does nothing, including under --check
— a game built on this server has its own docs, or none, and should not have
a priv/docs tree conjured for it. Pass -o PATH to write somewhere else
deliberately; an explicit path is always honoured.