It doesn't contain all the scripts, far from it, and it only provides documentation for lua (no ruby/python/c++).
It's the official repo. See the develop branch for new stuff. Also there is no python bindings (afaik) and c++ part is mostly comment-documented and i learned by examples/hanging out in the irc and bugging the people that know stuff. As for ruby bindings (I forget who wrote it) they are really similar to lua.
Biggest problem is that you need to know df very intimately to do stuff. And there is just so much to know... I mean
here is the data structures but still there are strange ways df is doing stuff. AFAIK there is no easy way. Also i should have written many tutorials and maybe someday i will until then i'll repeat myself: check other people scripts (especially the official repo ones, they might be nicer, more correct, etc...), bug people (especially in irc, though remeber timezones, and patience), play with gui/gm-editor - that is the thing i made for myself to check stuff out, and edit df on the fly and soon you'll be writing next great total conversion that allows you to build rockets and crash moons into enemy forts.