Add Podman 6 HLD

Adding a design document for Podman 6.  it is a high level design that tries to encompasses proposed changes to Podman and its runtime environment.  It should highlight major changes and features; but you will also find nitty detailed changes that have been idenitified.  It is quite possible things in this document will change but it allows for open, transparent communication.

Signed-off-by: Brent Baude <bbaude@redhat.com>

Author: baude