Add 'doc' and 'doc_clean' make targets for Effects lib

Running `make lib_doc` from the root was failing with this error.

> ...
> /Applications/Xcode.app/Contents/Developer/usr/bin/make	-C effects doc
> make[2]: *** No rule to make target `doc'.  Stop.
> make[1]: *** [doc] Error 2
> make: *** [lib_doc] Error 2

The problem was `libs/effects/Makefile` lacked the necessary 'doc' target.