docs(system-design): add distributed transactions guide

[shalshcode08]

Summary

• add a 10,308-word standalone guide for System Design Topic 11
• explain local and distributed transaction boundaries, two-phase commit protocol and recovery, sagas, orchestration, choreography, compensation, pivot transactions, and durable execution
• cover transactional outbox, inbox deduplication, CDC and polling relays, ordering, idempotency, ambiguous outcomes, reconciliation, and exactly-once boundaries
• include a complete order, payment, and inventory workflow with state transitions, failure matrix, recovery jobs, security boundaries, audit controls, performance estimates, testing, exercises, interview Q&A, cheat sheet, and official references
• register the guide in the System Design navigation manifest without changing styling or tests

Validation

• Prettier write and check: passed
• complete test suite: 22 passed, 0 failed
• production build: passed, 40 pages generated
• html-validate on source and generated site: passed
• git diff check: passed
• scope check: only the topic file and manifest.json
• exact July 2026 footer check: passed
• prohibited em dash check: passed
• raw code-arrow check: passed

Sources

Technical claims are grounded in official Jakarta Transactions, Java XA, PostgreSQL, Microsoft Azure Architecture Center, AWS Prescriptive Guidance, Debezium, Apache Kafka, CloudEvents, OpenTelemetry, and Temporal documentation.