Document ZMK-specific KConfig and DT settings

[joelspadin]

We've had a few people ask recently how to adjust the debounce period, and there are settings for it (e.g. https://github.com/zmkfirmware/zmk/blob/main/app/drivers/zephyr/dts/bindings/kscan/zmk%2Ckscan-gpio-matrix.yaml#L17 for matrix kscan) but they aren't described anywhere in the documentation.

It would be very helpful to have a place in the documentation for the ZMK-specific KConfig options and DT properties, but like we have for behaviors. Ideally we would generate this from the KConfig and bindings YAML files directly, but if that is too difficult, manually documenting some of the more commonly used things would be a good start.

We should also link to Zephyr's documentation for any options that are particularly useful for ZMK, like USB polling rate.

[joelspadin]

I've started working on manually documenting the existing settings and adding some documentation on how to set them.