docs/sphinx: Skeleton setup

docs/sphinx: Skeleton setup

Sphinx is a documentation system, which is popular for technical writing.  It
uses ReStructuredText as its markup syntax, and is designed for whole-project
documentation, rather than the misc assortment of individual files that we
currently have.

This is a skeleton setup which just enough infrastructure to render an empty
set of pages.  It will become better integrated into Xen's docs system when it
becomes less WIP.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>