Move htools-ganeti 2.3 design doc to doc/

I didn't know where to put in the TOC, so I added it after the ganeti
2.3 design doc.

Also, it seems that Sphinx is a bit limited in its list nesting
(compared to Pandoc), so I had to rework a bit the indentation (it
still doesn't generate good output, but…).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>