Where is the documentation for how to access raw markdown for a GitHub wiki page?
Does GitHub publish documentation about how to access the raw markdown source for a wiki page?
Yes, GitHub documents how to export wiki in a blog, when Wikis is released in 2010.
Each wiki is a Git repository, so you're able to push and pull them like anything else. Each wiki respects the same permissions as the source repository. Just add ".wiki" to any repository name in the URL, and you're ready to go.
In your mbigras/hello-world case, the command would be:
git clone https://github.com/mbigras/hello-world.wiki.git