we can't use cross-project links on the twisted web site while also allowing cross-project links in the distributed documentation

[radix]

We should deploy the docs to the web site by simply copying the contents of the "doc/" directory of the tarball to the web site somewhere, instead of moving all the children of that directory into disparate locations on the site. This would allow "naive" cross-project links ("../../core/howto/whatever.xhtml") to work.