Document required Jibchain network configuration

[AM-979]

Summary

Document the authoritative Jibchain Lighthouse configuration source and the exact directory that must be passed to --testnet-dir.

Changes

• identify https://github.com/jibchain-net/node as the required configuration source
• add clone and update commands
• verify ~/node/config/config.yaml exists
• distinguish the correct node/config path from the incorrect repository root
• show how to pass a custom absolute config path
• warn that the config and beacon endpoint must belong to the same Jibchain network
• clarify the default configuration table and CLI option description

Why

Without the Jibchain custom-network files, Lighthouse cannot construct or validate the correct chain context for the voluntary exit. Pointing --testnet-dir at the repository root instead of its config subdirectory also causes failures.