Document ways of signing a transaction

1. Core.Transaction.sign() using secret
2. Key.signParcel() using Keystore
3. Rpc.chain.sendTransaction() using CodeChain AccountProvider

Start by locating Core.Transaction.sign(), Key.signParcel(), and Rpc.chain.sendTransaction() in the SDK and read how each signing path is exposed. Document the three approaches, including secret-based signing, Keystore signing, and signing through the CodeChain AccountProvider; done means users can distinguish when to use each one.

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