Right now the markdown pages do not generate proper intra-document links so it wouldn't be a great tool - today - as a wiki.
Gitblit could be extended to support that, or perhaps ${reponame}-wiki.git, which I think GoogleCode uses. Not sure about Github... that might use -wiki.git repos too.
Alternatively, you can serve static html documentation on an orphan branch refs/heads/gh-pages. If this ref is present, it will display the "pages" link which will serve content directly from the ref like this:
-J