[RFC] Moving user's guide to ReStructuredText

[David Luposchainsky]

I really like the idea of having a readable raw format that renders nice. Editing
is awful, yes, but we don't have to do it that often. But what matters much more
to me, and in fact most GHC users, is searchability. I don't want to enter the
user's guide via Google, and grepping through HTML is not very fruitful.
A markdown-ish format gives us a lot of our usual tools to work with the manual.
Huge +1 from me.

David