docs: note canonical explorer/docs URLs and flag dead ones

Document that testnet.arcscan.app and docs.arc.network are the canonical
Arc Testnet explorer and docs, and that the dead explorer.testnet.arc.network,
explorer.arc.io, and docs.arc.io URLs should not be used.

Fixes #81

Author: memosr

README.md

@@ -64,6 +64,13 @@ To connect a wallet to Arc Testnet:
 > # {"result":"0x4cef52"}  =  5042002
 > ```
 
+> [!NOTE]
+> The canonical Arc Testnet block explorer is **https://testnet.arcscan.app** and
+> documentation lives at **https://docs.arc.network**. Older explorer URLs such as
+> `explorer.testnet.arc.network`, `explorer.arc.io`, and `docs.arc.io` are dead
+> (unreachable, login-gated, or 404) — do not use them in wallet `blockExplorerUrls`
+> configs or transaction links.
+
 ## Install and Run a Node
 
 ### Install