This reverts commit 5e8ff010a1.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
I have no idea if this is going to cause rendering problems, and it is fairly
hard to check. So let's just merge this, and if it github markdown processor
doesn't like it, revert.
