fix: use Blockscout verifier flags for Ink Sepolia contract verification

[schoolkamsergj]

Problem

The "Verifying Your Contract" section in the Foundry deployment guide referenced --etherscan-api-key for contract verification on Ink Sepolia. However, Ink uses Blockscout as its block explorer — not Etherscan. Using --etherscan-api-key fails with an unsupported chain error.

This was originally reported in #609.

Changes

1. Fixed contract verification command

Before:

forge verify-contract <ADDRESS> src/InkContract.sol:InkContract \
    --chain-id 763373 \
    --etherscan-api-key $BLOCKSCOUT_API_KEY

After:

forge verify-contract <ADDRESS> src/InkContract.sol:InkContract \
    --chain-id 763373 \
    --verifier blockscout \
    --verifier-url https://explorer-sepolia.inkonchain.com/api \
    --verifier-api-key $BLOCKSCOUT_API_KEY

2. Unified RPC URL variable name

The .env example used RPC_URL while foundry.toml referenced INKSEPOLIA_RPC_URL. Both now consistently use INKSEPOLIA_RPC_URL.

3. Clarified section heading

Updated the section intro to explicitly state that Ink uses Blockscout, not Etherscan, so developers understand why the flags differ from standard Ethereum tooling.

Testing

Verified against the Blockscout Foundry verification docs and the Ink Sepolia explorer endpoint https://explorer-sepolia.inkonchain.com/api.

Fixes #609