Add University of Nottingham's courses (haskell-infra#234)

* Use canonical link to directory (including slash)

Linkchecker complains otherwise.  Perhaps the behavior of the server
has changed?

* Add University of Nottingham's courses

site/community.markdown

@@ -55,7 +55,7 @@ Haskell infrastructure is mostly maintained by committees and groups of voluntee
 *   [Core Libraries Committee](https://github.com/haskell/core-libraries-committee)
 *   [GHC Steering Committee](https://github.com/ghc-proposals/ghc-proposals)
 *   [Hackage trustees](https://wiki.haskell.org/Hackage_trustees)
-*   [Haskell.org committee](/haskell-org-committee)
+*   [Haskell.org committee](/haskell-org-committee/)
 *   [Stackage maintainers](https://docs.haskellstack.org/en/stable/maintainers/team_process/)
 *   [Haskell admin team](https://github.com/haskell-infra/haskell-admins/): responsible for all `***.haskell.org` infrastructure, including this website.
 

site/documentation.markdown

@@ -45,6 +45,8 @@ Course material created by instructors
 *   [Stanford's cs240h](http://www.scs.stanford.edu/14sp-cs240h/)
 *   [Hendrix's CSCI 360](http://ozark.hendrix.edu/~yorgey/360/f16/)
 *   [University of Helsinki's Haskell MOOC](https://haskell.mooc.fi/)
+*   [University of Nottingham's introductory Haskell course](http://www.cs.nott.ac.uk/~pszgmh/pgp.html)
+*   [University of Nottingham's advanced Haskell course](http://www.cs.nott.ac.uk/~pszgmh/afp.html)
 
 ## Tutorials