[gudiri1]

Closes #3938

Adds a "Change the primary UART" section to the UART configuration page. It gives
model-specific guidance in one place:

• Raspberry Pi Zero W, Zero 2 W, 3, and 4: cross-references the existing disable-bt
  and miniuart-bt procedures for making UART0 primary.
• Raspberry Pi 5, 500, 500+, and Compute Module 5: explains that the primary UART is the
  dedicated debug header by default, and how enable_uart=1 routes the console to GPIO 14/15.
• All other models: UART0 is already primary; no change needed.

Also adds a troubleshooting note for the common "my serial console disappeared after
switching to a Raspberry Pi 5" case, and adds anchors to the two existing overlay
procedures so they can be cross-referenced.

[katshann-raspberry]

@gudiri1 Thank you for your suggestion.

@pelwell Please can you take a look at this PR and advise on the correctness of the content before we take it to publish. Cheers!

[pelwell]

The information looks correct and well written. I'd probably move "when no cable is connected to the dedicated debug header, this routes the console to GPIO 14 and 15" to after the example, but that's just personal taste.

I leave it up to the documentation team to decide if this is actually wanted.

[gudiri1]

Thanks @pelwell! Good suggestion — I've moved that clause to after the example, which reads more cleanly and removes a small repetition. Updated.