Generate a ``sitemap.txt`` from the collected HTML page links.

def build_finished(app, exception):
    ''' Generate a ``sitemap.txt`` from the collected HTML page links.

    '''
    filename = join(app.outdir, "sitemap.txt")

    links_iter = status_iterator(sorted(app.sitemap_links),
                                 'adding links to sitemap... ',
                                 'brown',
                                 len(app.sitemap_links),
                                 app.verbosity)

    try:
        with open(filename, 'w') as f:
            for link in links_iter:
                f.write("%s\n" % link)
    except OSError as e:
        raise SphinxError('cannot write sitemap.txt, reason: %s' % e)