Builds the menu used for the current ``DocumentWriter``.

def _build_menu(docs):
    """
    Builds the menu used for the current ``DocumentWriter``.
    """

    paths = []
    current = docs.filename
    while current != docs.root:
        current = current.parent
        paths.append(current)

    for path in reversed(paths):
        docs.add_menu(path.stem.title(), link=path / 'index.html')

    if docs.filename.stem != 'index':
        docs.add_menu(docs.title, link=docs.filename)

    docs.end_menu()