docs: def vtt_formatter(subtitles, padding_before=0, padding_after=0):
      """
      Serialize a list of subtitles according to the VTT format, with optional time padding.
      """
      text = srt_formatter(subtitles, padding_before, padding_after)
      text = 'WEBVTT\n\n' + text.replace(',', '.')
      return text