* rename and restruct ProofInstruction::VerifyCloseAccount to ProofInstruction::VerifyZeroBalance
* rename `PubkeySigmaProof` to `PubkeyValidityProof`
* refactor equality proofs into separate modules
* rename and restructure ProofInstruction::VerifyWithdrawWithheld to ProofInstruction::VerifyCiphertextCiphertextEquality
* add and reorg documentation for the proof instructions
* generalize ciphertext-ciphertext equality proof data constructor
* Update zk-token-sdk/src/instruction/ctxt_ctxt_equality.rs
Co-authored-by: Jon Cinque <joncinque@pm.me>
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Co-authored-by: Jon Cinque <joncinque@pm.me>