[racket] Module docs

I think the problems that you saw are now fixed in the nightly build.

Running `raco setup' on a refresh install still rebuilds a bit of
"mysterx". That's a timestamp issue with "mxmain.dll" and the way we
create the distribution, and maybe we can improve that. More
importantly, `raco setup' doesn't try to rebuild all documentation or
get confused about multiple paths for documents.

I tried a git checkout of 

 https://github.com/blippy/tacc/tree/master/scheme/carali

and things seemed to work right. I didn't use `mklink' (I was using
Windows XP), and I instead used the new `raco link' command. I think
`raco link' is more convenient, but `mklink' should work, too. After
creating the collection link, `raco setup' built your docs and added
them to the index as it should, and a second `raco setup' left
everything in good shape.

For the next person starting out, I think the new Guide section on
modules explains better how to get started with modules. It doesn't yet
discuss documentation, though, and I plan to work on the section more.