There is already a pin-switch in the door that triggers the lights.
You can use that switch to trigger a relay which would trigger a start-up/shut-down controller for the PC.
The problem with the trigger from opening the door would be if you are in a situation that requires opening/closing the door multiple times in a short period of time, such as if you are picking someone up, or you stop to drop off a package or something similar.
For this reason, SSDCs are triggered by the car's ignition, so when you turn the key, the PC starts up. When you turn the key off, the PC shuts down.
It's a lot simpler set up than your door idea.