Che 7 docs left nav doesn't scroll when it's longer than the screen

In addition to the fact that search doesn't work (RHDEVDOCS-1267 )... if you expand too many sections of the left nav on https://www.eclipse.org/che/docs/che-7 , you can't get to the bottom of the nav anymore. Or if you click the 'Expand all' link... there's no way to unexpand them other than reloading the page.

If we want the docs to be useful we need to make the nav scrollable.