Documentation: PM: sleep: Document system-wide suspend code flows

Add a document describing high-level system-wide suspend code flows
in Linux.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/admin-guide/pm/system-wide.rst

@@ -8,3 +8,4 @@ System-Wide Power Management
    :maxdepth: 2
 
    sleep-states
+   suspend-flows