Why not to use the GitHub Wiki feature.
Wiki pages cannot be sorted into folders
and thus they inevitably become a mess.
Relative links to for example images don't work
on wiki pages, this means you will end up with
markdown cluttered full of long absolute links.
Wikis have a separate version history
and cannot easily be contributed to
as you would to a normal repository.
Simply create a /Documentation/ folder.
If you want to separate the documentation completely
you may want to create a Documentation branch.
In case you have a lot of documentation and
yourrepository is part of an organization, you
could also create a dedicated repository for it.