docs: add ZO_HTTP_REAL_IP_SOURCE environment variable

[hengfeiyang]

Summary

Documents the ZO_HTTP_REAL_IP_SOURCE environment variable in the Network and Communication section of the environment variables reference.

This variable controls how OpenObserve resolves the real client IP when running behind a proxy or load balancer. Details (default value, supported sources, and the spoofing caveat) were taken directly from the OpenObserve source (config.rs env_config and the real_ip.rs resolver).

• Default: XEnvoyExternalAddress,XRealIp,RightmostXForwardedFor
• Sources are tried in order, first match wins, with the TCP peer as the final fallback.
• Lists all supported values (Envoy/Istio, nginx, AWS ALB/CloudFront, Cloudflare, Fly.io, etc.) and the note that only trusted upstream proxies should be listed.

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