[GHC] #14023: Split up glasgow_exts.rst

[GHC]

#14023: Split up glasgow_exts.rst
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Documentation  |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The `docs/users-guide/glasgow_exts.rst` is crazy large. This makes it
 difficult to navigate both while editing and reading. It should probably
 be split up into multiple files along the top-level headings.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14023>
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