Fix ruby-doc.org links that reference the old site structure

[dazuma]

The generator generates some ruby-doc.org links of the form https://ruby-doc.org/stdlib/libdoc/.... These no longer work and have been replaced with https://ruby-doc.org/current/stdlibs/.... Any instances of that old form need to be fixed in the templates and generator.

Once this is done, a hack in the owlbot postprocessor called fix_rubydoc_org_links should be removed.