Tests

docs: add docs for `starknet-signers` #1380

Job	Run time
no-std build	44s
WASM tests	3m 51s
Unix tests (ubuntu-latest, stable)	2m 3s
Unix tests (ubuntu-latest, nightly)	1m 50s
Unix tests (macos-12, stable)	3m 41s
Unix tests (macos-12, nightly)	2m 57s
Windows tests (stable, x86_64-pc-windows-gnu)	4m 24s
Windows tests (stable, x86_64-pc-windows-msvc)	3m 55s
Windows tests (nightly, x86_64-pc-windows-gnu)	5m 21s
Windows tests (nightly, x86_64-pc-windows-msvc)	5m 5s
	33m 51s

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