How to change the primary UART of a Raspberry Pi

[nllho]

Related to #3239.

Context:

• The primary UART is where the Linux console appears.
• On most Raspberry Pis, the primary UART appears on the GPIO header.
• For Raspberry Pi 5, the primary UART appears on the Debug header.

Use case:
When switching to Pi 5 from older versions, people get confused because the primary UART has changed, and they do not see the console as expected. This confusion may persist into the future with new Pi models and as people upgrade their hardware.

Current state:
The bullet points above already exist in the documentation. However, we do not currently document how to change the primary UART of a Raspberry Pi.

Possible docs solutions:

1. Create a troubleshooting section for "the console does not appear" somewhere under initial configuration or UART configuration. This troubleshooting section could be expanded with multiple solutions, including the information here about using the correct primary UART for a given Raspberry Pi model.

2. Add "Changing the primary UART" to https://www.raspberrypi.com/documentation/computers/configuration.html#configure-uarts. This comment by Phil explains how to change the primary UART of the Pi 5 to the GPIO header. If we create this section, we would need to confirm instructions for all Pi models.