Convert site documentation to xdoc format

The current Confluence markup doesn't support comments (which annoyingly means we can't include a license header), and doesn't seem to have the same level of support as Apt or Xdoc in Doxia. There is a converter tool to convert from Confluence (http://maven.apache.org/doxia/doxia-tools/doxia-converter/usage.html) - and it seems to do a decent job - so this shouldn't be a lot of work.