Even if building a Ruby with documentation, there's no reason for its
baseruby to waste time building its own documentation as well.

This allows getting access to Ruby documentation through ri by doing

    nix-shell -p ruby ruby.devdoc

or by installing the ruby.devdoc package.

A setup hook will add a shim to LOAD_PATH to point ri to the devdoc
output instead of out.

Now that RI docs are installed to a seperate output, we don't need to worry
about bloating the main output, so it's reasonable for this to be enabled by
default.

ruby: install ri docs to devdoc output