docs: add docs for `starknet-contract` #620

Job	Run time
Build C++ integration example	1m 9s
Build starknet-curve features	18s
Build starknet-crypto features	32s
Build starknet-core features	1m 27s
Build starknet-providers features	1m 19s
Build starknet-signers features	53s
Build starknet-accounts features	50s
Build starknet-contract features	52s
Build starknet-macros features	35s
Build starknet features	1m 44s
	9m 39s

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