doc/release-notes: v0.56.1

Signed-off-by: Sage Weil <sage@inktank.com>
doc/install: new URLs for argonaut vs bobtail

Also restructure the document a bit to make the choice of packages more
clear.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph