docs: def atx_header_sub(match):
      """Substitutes atx headers (headers defined using #'s)."""
      level = len(match.group(1))
      title = match.group(2)
  
      id_class = ''
      ids = match.group(3) if match.group(3) else ''
      id_match = re.search('#([\w-]+)', ids)
      if id_match:
          id_class += ' id="' + id_match.group(1) + '"'
      classes = ' '.join(re.findall('\.([\w-]+)', ids))
      if classes:
          id_class += ' class="' + classes + '"'
      return '\n<h{0}{2}>{1}</h{0}>\n'.format(level, title, id_class)