doc: add texinfodocs and infodocs targets

Sphinx supports generating Texinfo sources and Info documentation,
which can be navigated easily and is convenient to search (via the
indexed nodes or anchors, for example).  This is basically the same as
1f050e904dd6f2955eecbd22031d912ccb2e7683, which was recently applied
to the Linux kernel.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>