Convert Kea docs to reStructuredText / Sphinx

I would like to convert the Kea docs from DocBook to reStructuredText. We can easily publish in html and .pdf format, as we do today, but we can also easily display the docs for multiple Kea versions, as on readthedocs. We can also post our docs on readthedocs, and on kea.isc.org where they can be an easy on-line resource.

If sweng will do the initial steps of creating the branch and converting the files to reStructredText format, Suzanne will do the manual markup fixes required. (@godfryd estimates about 20% of the file will have to be re-marked up, so this is not insignificant.)