docs: Add a diagram for the internals of sysupdate

docs: Add a diagram for the internals of sysupdate

I had to sketch this out before I could get the internals of
systemd-sysupdate straight in my head, particularly around how an
`UpdateSet` points to one `Instance` from each of a set of `Resource`s,
and those `Instance`s are either all sources or all targets.

Hopefully this is useful to the next person to look at the code.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
(cherry picked from commit a05439097f7c188d27f58241b8621b2132a780e1)
(cherry picked from commit 877b9f9a0db1f2cd9456856c24944ed20aff8c6a)
(cherry picked from commit b14faca811f24fb6b88ae4c774e627dda54bbd29)