Quite a few people have struggled with setting the correct
`chain-length` for their `led_strip`. For some, this is their first time
needing to change a pre-defined devicetree property, and so they aren't
familiar with the technique. I commonly see folks adding a duplicate of
the entire `*.overlay` file with only the `chain-length` value changed.
Having clear documentation for this specific application of the property
override technique could help forestall these problems, and give those
of us helping out in Discord something to easily reference.
* Update underglow.md
Updated the documentation according to my current understandring of how to pin-mapping works, also split the chapter into a nrf-section and a non-nrf-section since that information isn't applicable to both
* Ran prettier and updated pin reference according to suggestion
