docs: document signer and testing strategies

[mattsse]

Summary

• replace the stale signer page with a current trait, feature, integration, and wallet-selection guide
• map local, mnemonic, keystore, hardware, cloud KMS, YubiHSM, and Turnkey signers to their features and examples
• add a testing strategy covering mocked transports, local Anvil/Geth/Reth nodes, pinned forks, and live RPC smoke tests
• add secret handling, chain-ID verification, CI isolation, and determinism guidance

Why

The existing signer guide uses pinned source links and incomplete snippets, covers only three implementations, and does not explain signer selection or safe configuration. Testing capabilities are discoverable only by finding individual examples, so users lack guidance on when to mock, spawn a node, fork state, or call a live endpoint.

Impact

Users and agents can choose the correct signer feature and wallet behavior from one page, and application tests can progress from fast deterministic mocks to explicit integration environments without silently depending on public infrastructure.