Add 'orphan' to metadata for partials

    :type app: sphinx.application.Sphinx
    :type doctree: docutils.nodes.document

def doctree_read_handler(app, doctree):
    """
    Add 'orphan' to metadata for partials

    :type app: sphinx.application.Sphinx
    :type doctree: docutils.nodes.document
    """
    # noinspection PyProtectedMember
    docname = sys._getframe(2).f_locals['docname']
    if docname.startswith('_partial'):
        app.env.metadata[docname]['orphan'] = True