docs: fix link to Pyrex embedding tutorial (GH-3550)
docs: fix link to Pyrex embedding tutorial (GH-3550)

* Updated URL for the "embedding Python" link in external_C_code.rst
  in the userguide.
* The freenet.org.nz domain has clearly expired at some point and now
  the original link goes spam links, but there are copies in the
  Wayback Machine.
* The updated (fixed) URL uses the most recent archive for the
  original page (author: June, 2004; web archive: Feb, 2012).
da-woods/cython,da-woods/cython,cython/cython,scoder/cython,scoder/cython,cython/cython,da-woods/cython,da-woods/cython,scoder/cython,cython/cython,scoder/cython,cython/cython