[racket] package server + making docs available early

I've used gh-pages for bigger libraries and for Fear of Macros.
(Although, since the latter is more like a very small book, when I
have time I want to check out Pollen instead!)

For smaller/simpler libraries I find futzing with gh-pages enough of a
PITA that instead I just do:

    scribble --markdown manual.scrbl

The resulting manual.md file is directly viewable on GitHub on a
normal branch like master. Example:
https://github.com/greghendershott/http/blob/master/http/manual.md

It is definitely not as fully-featured as Scribble HTML output. And
the "blue boxes" look too similar to examples, since both are just
markdown code blocks.

However it exceeds the "give me convenience or give me death"
threshold -- at least I end up doing better docs than nothing but a
readme.

Having said all that, I wish it were possible to reincarnate the
doc-hosting portion of the Planet server, for packages.