Day 7 - Add Nitish Songara's DevOps stack

[sonu7089]

Adds the ## My Stack section to content/experts/nitish-songara.md as part of the Hacktoberfest Day 7 challenge (branched from the Day 1 branch hacktoberfest/add-nitish-songara).

Changes

• Added ## My Stack covering CI/CD & Version Control (Git, GitHub Actions, ArgoCD), Infrastructure & Cloud (Terraform, AWS), Containers & Orchestration (Docker, Kubernetes), Monitoring & Observability (Prometheus, Grafana, CloudWatch, OpenTelemetry), ### Why This Stack, and ### One Thing I'd Change.
• Validated with pnpm test tests/expert-validation.test.ts.