14IMH9: Add & modify workarounds, add notes about 14imh9

[pokon548]

Description of changes

• Add workaround for i915 0000:00:02.0: [drm] GT0: Resetting chip for stopped heartbeat on rcs0
• Use systemd.sleep.settings.Sleep because nixos-unstable deprecates systemd.sleep.extraConfig
• Add note about how to turn off rc6. It will cause GPU hangs and currently no mainlined workaround is applicable, unfortunately. So users have to manually turn it off in various ways
• Add note about dealing with VK_ERROR_DEVICE_LOST

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input