doc: develop: west: alias: add example for auto-excluding platforms

Add an example west alias for automatically excluding certain platforms
when running Twister via west.

This is especially useful for excluding the 32-bit native_sim target when
running on hosts systems without a 32-bit host C library
(i.e. Linux/AArch64).

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>