Fix #50: Fix up links to the original docs
The links for the original cmucl design pdf was a relative path which doesn't exist on the new cmucl site. Make the links point to locations on cmucl.org so that they work on cmucl.common-lisp.dev.
These files still exist on https://common-lisp.net/project/cmucl/downloads/doc/, but I don't know how to add such things to cmucl.common-lisp.dev. So I'm taking the easy way and pointing to cmucl.org which syncs with this directory anyway.