Generate URL for path in the Taskcluster docs.

def docs(root_url, path):
    """Generate URL for path in the Taskcluster docs."""
    root_url = root_url.rstrip('/')
    path = path.lstrip('/')
    if root_url == OLD_ROOT_URL:
        return 'https://docs.taskcluster.net/{}'.format(path)
    else:
        return '{}/docs/{}'.format(root_url, path)