GB: Prefer "modern" Pan Docs for bibrefs

[ISSOtm]

The GB article cites various sources, including the Ultimate GB Talk and GBDev Wiki; however, most of that info can be found in https://gbdev.io/pandocs, which is maintained and thus more up to date, and I think also more accessible. (And cites primary sources such as the Talk as well.)

Sources that can be updated:

• The Ultimate Game Boy Talk → CPU Comparison with Z80
• Programming info → Memory Map (could also supersede the "GameBoy memory map"
• Graphics emulation → Graphics Overview
• Gameboy sound hardware → Audio Overview (note that the banner at the top does link to "modern" Pan Docs)