Documentation: power: Add document on debugging shutdown hangs

Documentation: power: Add document on debugging shutdown hangs

If the kernel hangs while shutting down, ideally a UART log should
be captured to debug the problem.  However if one isn't available,
users can use the pstore functionality to retrieve logs.

Add a document explaining how this works to make it more accessible
to users.

Tested-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/20251025004341.2386868-1-superm1@kernel.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>