Guides tree rewrite

As discussed at spreeconf, I think we should pull the documentation into spree/guides rather than keeping it as a separate repo.

This allows us to make sure that, as changes are accepted, the guides are properly updated as well. This is especially important for "Upgrade Guides", as we can ensure that breaking changes are explained and handled better than they have been in the past.

The drawbacks are increased repo size, and a slightly higher list of requirements on incoming PR's (reasonable documentation changes), but I think they're important for the project going forward.