Add description for COPPER

[tomaz]

https://wiki.specnext.dev/Copper

https://gitlab.com/thesmog358/tbblue/blob/master/docs/extra-hw/copper/COPPER-v0.1c.TXT

https://docs.google.com/document/d/1nhBNre5ePgelGfcB0KRmut1D3lUlk8R1IFj4MAmAiK0/edit?fbclid=IwAR2jNnqozTZZIh9-6mRu9xT4CAv2fwHCwWjh0DxXabGonu5LjBMflZbOkRo

You can update Next interrupts register $64 with Copper related functionality (currently omitted as Copper is not described in the book). Probably in that case, $64 would be moved to Copper chapter anyway, so only link to it would remain in interrupts.

[tomaz]

Copper is now described 🎉