Create in the build directory an html file with a redirect,
        for every row in REDIRECTS_FILE.

def _add_redirects(self):
        """
        Create in the build directory an html file with a redirect,
        for every row in REDIRECTS_FILE.
        """
        html = '''
        <html>
            <head>
                <meta http-equiv="refresh" content="0;URL={url}"/>
            </head>
            <body>
                <p>
                    The page has been moved to <a href="{url}">{title}</a>
                </p>
            </body>
        <html>
        '''
        with open(REDIRECTS_FILE) as mapping_fd:
            reader = csv.reader(mapping_fd)
            for row in reader:
                if not row or row[0].strip().startswith('#'):
                    continue

                path = os.path.join(BUILD_PATH,
                                    'html',
                                    *row[0].split('/')) + '.html'

                try:
                    title = self._get_page_title(row[1])
                except Exception:
                    # the file can be an ipynb and not an rst, or docutils
                    # may not be able to read the rst because it has some
                    # sphinx specific stuff
                    title = 'this page'

                if os.path.exists(path):
                    raise RuntimeError((
                        'Redirection would overwrite an existing file: '
                        '{}').format(path))

                with open(path, 'w') as moved_page_fd:
                    moved_page_fd.write(
                        html.format(url='{}.html'.format(row[1]),
                                    title=title))