docs(system-design): add observability and SRE guide

[shalshcode08]

Summary

• add a complete standalone guide for System Design Topic 13
• explain logs, metrics, traces, profiles, events, context propagation, sampling, cardinality, retention, and telemetry pipelines
• cover RED, USE, golden signals, SLIs, SLOs, SLAs, error budgets, burn-rate alerts, availability math, tail latency, queueing, and Little's Law
• teach capacity models, forecasting, headroom, load, stress, soak, scalability, chaos, and recovery testing
• provide incident command, severity, communications, runbooks, on-call readiness, blameless postmortems, exercises, interview answers, a revision cheat sheet, and primary official references
• include a complete evidence-led investigation of high order-creation latency across multiple distributed components
• add the page to the System Design navigation manifest in the authoritative order available on the branch base

Why

TOPICS.md defines Observability, SRE, capacity planning, and incident response as System Design Topic 13. This guide provides in-depth, easy-language learning and interview preparation while staying language-neutral and using only an approved Go example where concrete code improves understanding.

Scope

Only these files change:

• src/content/systemdesign/observability-sre-capacity-planning-incident-response.html
• manifest.json

No tests, styles, shared design, or TOPICS.md files were changed.

Validation

• npx prettier --write src/content/systemdesign/observability-sre-capacity-planning-incident-response.html manifest.json
• npm test
• npm run build
• HTML validation and Prettier checks included in the complete test suite
• git diff --check
• exact required footer check
• changed-file scope check
• no new em dash check
• raw -> and >= checks for HTML-safe diagram and pseudocode content

References

Technical claims are grounded in current primary official OpenTelemetry specifications and documentation, W3C Trace Context and Baggage specifications, Prometheus practices, Google SRE books and workbooks, NIST SP 800-61 Rev. 3, and Oracle Java Flight Recorder documentation.