Transcripts don't support markdown

[shirgoldbird]

Though the docs say transcripts support markdown, this wasn't the case when I tested. I added support by modifying the get_transcripts function in build_site.py: (lines 231-233)

  with open(path, "rb") as f:
      transcripts[language] = f.read().decode("utf-8").replace("\n", "<br>\n")
      transcripts[language] = MARKDOWN.convert(transcripts[language])

Happy to open a PR for this change but wanted to confirm first this is a bug.