Define and use `extlinks` link shorteners in user's guide

This patch defines 3 short linkeners,

    extlinks = {
      'issue': ('https://github.com/haskell/cabal/issues/%s', '#'),
      'ghc-wiki': ('http://ghc.haskell.org/trac/ghc/wiki/%s', ''),
      'ghc-ticket': ('http://ghc.haskell.org/trac/ghc/ticket/%s', 'GHC #'),
    }

See http://www.sphinx-doc.org/en/stable/ext/extlinks.html for more
information as well as this diff for examples on how to use them.