docs: multiple Arc Testnet block explorer URLs are dead — testnet.arcscan.app is the live explorer

[osr21]

Summary

Several Arc Testnet block explorer URLs that appear in official documentation, MetaMask blockExplorerUrls configs, and community tooling are returning 404s or are completely unreachable. This breaks transaction links, debugging workflows, and wallet chain configurations.

Dead URLs

URL	Status
https://explorer.testnet.arc.network	❌ Unreachable / times out
https://explorer.arc.io	❌ Requires team login — not publicly accessible
https://docs.arc.io	❌ Returns 404 (linked from some external resources)

Working URL

URL	Status
https://testnet.arcscan.app	✅ Live and functional
https://docs.arc.network	✅ Live and functional

Impact

1. Wallet configuration — When adding Arc Testnet to MetaMask via wallet_addEthereumChain, the blockExplorerUrls field is set to the dead URL. MetaMask then shows broken explorer links for all transactions.
2. Documentation — Developers following older guides get dead links when trying to verify transactions.
3. PR docs: update outdated arc.network domain references to arc.io #70 — There is an open PR attempting to update domain references in this repo, but it appears to be going in the wrong direction (replacing arc.network with arc.io), which would make things worse.

Suggested Fix

Update all references to use testnet.arcscan.app as the canonical testnet block explorer URL, and docs.arc.network for documentation links. These should also be reflected in any official chain configuration examples in this repo.