ruby: install ri docs to devdoc output

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.

pkgs/development/interpreters/ruby/rbconfig.rb

@@ -0,0 +1,25 @@
+# This is a shim around whatever real rbconfig.rb is in the LOAD_PATH,
+# so that RbConfig::CONFIG["ridir"] can be overridden to point to the
+# custom location of the ri docs, without the main derivation having
+# those docs in its closure.
+
+MY_PATH = File.realpath(__FILE__)
+
+candidates = $LOAD_PATH.map { |dir| File.join(dir, "rbconfig.rb") }
+
+# First, drop everything _before_ this file in the LOAD_PATH, just on
+# the off-chance somebody is composing shims like this for some reason.
+candidates.drop_while { |c| !File.exist?(c) || File.realpath(c) != MY_PATH }
+
+# Now, the wrapped rbconfig.rb is the next rbconfig.rb in the LOAD_PATH
+# that isn't this same file. (Yes, duplicate LOAD_PATH entries are a
+# thing we have to deal with.)
+next_rbconfig = candidates.find { |c|
+  File.exist?(c) && File.realpath(c) != MY_PATH
+}
+
+# Load the wrapped rbconfig.rb
+require next_rbconfig
+
+# Now we have RbConfig, and can modify it for our own ends.
+RbConfig::CONFIG["ridir"] = File.expand_path("../../../share/ri", __dir__)