docs(contracts): document Arcscan Etherscan-compatible contract verification

[kutluhaneth46]

Summary

• Fixes the DX gap behind bug: testnet.arcscan.app contract verification API rejects all standard Blockscout formats #210: Arcscan's /api verify endpoint is Etherscan V1 compatible, not Blockscout-native. Requests that send addressHash / name / contractSourceCode get a misleading Missing codeformat field response even when codeformat is present.
• Document working Foundry and Hardhat verify flows against https://testnet.arcscan.app/api.
• Register an arc_testnet entry in foundry.toml so forge verify-contract can target Arc Testnet.

Verification

Reproduced both shapes against live Arcscan:

# Blockscout-native fields -> {"message":"Missing codeformat field","status":"0"}
# Etherscan V1 fields     -> {"message":"OK","status":"1", ...guid...}

Also confirmed:

forge verify-contract 0x5294E9927c3306DcBaDb03fe70b92e01cCede505 \
  contracts/src/memo/Memo.sol:Memo \
  --chain 5042002 \
  --verifier custom \
  --verifier-url https://testnet.arcscan.app/api \
  --verifier-api-key empty
# -> already verified (end-to-end API path works)

Notes

Hardhat still needs an etherscan.customChains entry for chain 5042002 (tracked separately if not yet on main). This PR documents the requirement and lands the Foundry side.

Test plan

• Reproduce bug: testnet.arcscan.app contract verification API rejects all standard Blockscout formats #210 Blockscout-native vs Etherscan field-name behavior against live Arcscan
• forge verify-contract against genesis Memo reaches Arcscan successfully
• Maintainer skim of contracts/README.md + foundry.toml [etherscan] entry