GenerateManpage: Drop all dependencies except base and invoke in CI

pandoc's API changes often enough that distros like Debian were patching
our GenerateManpage.hs to work with their version of pandoc, and it
doesn't build against any Stackage LTS except the recently released
LTS-17. Also, building pandoc from source takes quite some time and
resources.

But for what benefit? We're not using any special pandoc functionality
whatsoever. It's just that it was all in Haskell and the entire build
was orchestrated by cabal, but is all that complexity and resource
consumption worth it? I think not.

(Frankly, this whole thing was just a massive waste of time as the help
text in Config.hs isn't generated at all, so we still need to do this
manually. And then, the default key-bindings in core are unlikely to
change ever again.)

Let's simplify this:

* drop all dependencies except base and just run it through runhaskell

* add a Makefile and GH Actions workflow that invokes this after push

* only ship the results in release tarball, not the scripts which are
  considered our dev infrastructure

Closes: https://github.com/xmonad/xmonad/issues/283
Related: https://github.com/xmonad/xmonad/pull/260
Related: https://github.com/xmonad/xmonad/pull/261
Related: https://github.com/xmonad/xmonad/pull/265
Related: https://github.com/xmonad/xmonad/pull/266
Related: https://github.com/xmonad/xmonad/pull/267
Related: https://github.com/xmonad/xmonad/pull/268

man/Makefile

@@ -0,0 +1,11 @@
+.PHONY: all
+all: xmonad.1 xmonad.1.html
+
+xmonad.1.markdown: xmonad.1.markdown.in
+	(cd .. && util/GenerateManpage.hs) <$< >$@
+
+xmonad.1: xmonad.1.markdown
+	pandoc --from=markdown --to=man --standalone --output=$@ $<
+
+xmonad.1.html: xmonad.1.markdown
+	pandoc --from=markdown --to=html --standalone --table-of-contents --output=$@ $<