Document VPC egress gateway observability

[zhangzujian]

What this PR does

Documents the native observability support introduced by kubeovn/kube-ovn#7116:

• interface and low-cardinality conntrack NAT metrics;
• per-gateway Service and ServiceMonitor discovery;
• versioned JSON Lines NAT flow logs written to the observer container standard output;
• API fields, filters, rate limiting, hot reload behavior, security context, and operational limits;
• Kubernetes 1.29 and explicit CRD upgrade requirements.

Related issue

• [Feature Request] Add VpcEgressGateway Prometheus metrics and sidecar scraping support kube-ovn#7092

Validation

• markdownlint on the changed English Markdown files
• English punctuation check
• git diff --check

[zbb88888]

文档缺失了中文部分？