Add `generic-array` migration guide for `Concat` and `Split` #106

Job	Run time
clippy	12s
careful	44s
doc	22s
build (1.65.0, armv7a-none-eabi)	3m 4s
miri (x86_64-unknown-linux-gnu)	28s
build (1.65.0, thumbv7em-none-eabi)	3m 4s
miri (s390x-unknown-linux-gnu)	42s
build (1.65.0, wasm32-unknown-unknown)	3m 8s
rustfmt	5s
build (stable, armv7a-none-eabi)	8s
test (1.65.0)	3m 2s
build (stable, thumbv7em-none-eabi)	9s
test (stable)	10s
build (stable, wasm32-unknown-unknown)	19s
	15m 37s

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