Improve serial number and revision code documentation

[nathan-contino]

• Closes Best practice for revision code usage #3509
• Distribution-agnostic device tree values sourced from independent testing and https://github.com/torvalds/linux/tree/master/arch/arm/boot/dts/broadcom
• Cleans up wording and introduces a new distribution-agnostic section within revision code best practices
• Updates serial number documentation to account for the nonzero first eight digits of the Pi 5 serial number output (and potential future models where this might apply as well)