I like the idea of describing Arc 3.1 in the main docs. This way the docs can describe the Arc features people have already strived to keep consistent in projects like Jarc, Rainbow, Arcueid, and Arc/Nu. If other libraries or language variants need their own subpages, those can spring up as necessary. :)
A project like this should probably use an "arclanguage.github.io" repo rather than some other repo's gh-pages branch.
The arcfn static site generator may lead to some hassle. The generated website artifacts need to be pushed to the master branch, so they'll be the first thing someone sees when they delve into the repo to make a wiki edit. GitHub goes out of its way to make it easy to edit a repo as though it's a wiki, but that method of update won't trigger a very expressive build language, let alone anything that supports an Arc-based generator. Unless we have another server that does builds in response to a GitHub post-receive hook, in which case why not host the website there instead?
To use a custom generator, or to use Jekyll directly... both of these choices seem to have problems.
Another option would be to do keep Arc code inside <script> tags and run it with something like Rainbow.js, but search engines probably wouldn't index any content generated this way.
Hopefully someone here will be more optimistic about these ideas than I am. ^_^