I thought it was possible to link to external .md files from Gitits' wiki (.md) pages but it seems that whatever file I want to link to has to be committed in the Gitits' Git repo of wikidata directory.
Is there no way to achieve this?
So I found that out myself.
You just need to create a symlink to the file you want to show on the wiki and commit it to the
wikidata/ repo. That way Gitit will display the page with whatever content the symlink points to.
Also note that any change made to the content of the file/page through the wiki web interface will propagate to the linked file. BUT no commit will be made in either gitit or symlinked file repo (if that happened to be one); you'll just end up with edited file with no error shown.