move manual cabal install into its own page, and simplify it

doc/install/cabal.mdwn

@@ -0,0 +1,15 @@
+As a haskell package, git-annex can be installed using cabal. For example:
+
+	cabal update
+        cabal install git-annex --bindir=$HOME/bin
+
+The above downloads the latest release and installs it into a ~/bin/
+directory, which you can put in your PATH.
+
+But maybe you want something newer (or older). Then [[download]] the version
+you want, and use cabal as follows inside its source tree:
+
+	cabal update
+	cabal configure
+	cabal build
+	cabal install --bindir=$HOME/bin