mwext-VisualEditor-doc: Fix mw-setup-extension and modules path.

The former installed MediaWiki allright, but it didn't include
VisualEditor from LocalSettings.php, which meant that, though the
maintenance script worked, it didn't output anything useful as
the ResourceLoader modules of VE weren't registered.

Also fixing the relative path to modules. Since we only rsync the
docs/ directory, it couldn't find the ../modules directory.
Fixing by syncing that into a subdirectory.

Change-Id: Icfc14882995b3178a029943f317aa196d3de8635