Add documentation around pallet coupling (#3542)

substrate.io deprecation companion:
https://github.com/substrate-developer-hub/substrate-docs/pull/2139
pba-content companion:
https://github.com/Polkadot-Blockchain-Academy/pba-content/pull/978

partially inspired by:
https://github.com/paritytech/polkadot-sdk/issues/3535



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Co-authored-by: Ankan <[email protected]>