[codex] sync docs links and SDK examples

[haochencheng]

Summary

• Promote production docs links as the primary GitHub discovery path while keeping staging as the sandbox/testnet runtime.
• Refresh README, llms, lightweight docs indexes, and first-call examples for the current SDK conventions.
• Add official Go, Java/JVM, and .NET SDK guide links without copying full SDK docs into this lightweight funnel repo.

Validation

• Ran stale-pattern scans for old package/env/domain conventions.
• Ran git diff --check.
• Link checks: staging docs URLs returned 200; official SDK GitHub guide links returned 200; production docs URLs currently return 404 and should be treated as a release blocker per the docs sync plan.