Automate text-to-XHTML translation of Tor user manual

From the README:

3. Convert the English version of the user manual to XHTML:
   
    $ pandoc en/tsum.text -o short-user-manual_en.xhtml

    You need to update the xhtml file with header, footer and base64
    representation of how-tor-works.png.

That step should be automated.

(More generally, the whole build process for all languages should be automated.)