docs: add ecosystem architecture and contributor onboarding guide

[PragyaTripathi990]

Problem

New contributors to AMRIT face a steep onboarding curve because the README jumps straight into repo links without explaining the ecosystem, architecture, or how to start services in the correct order. This causes repeated questions in issues and slow ramp-up for GSoC/DMP applicants.

Related issues: #111

What this PR adds

A new "Understanding the AMRIT Ecosystem" section between "Getting Started" and "Repositories", covering:

• Service lines table — HWC, MMU, Helpline 104/1097, ECD, FLW, Scheduler with brief descriptions
• Architecture overview — Angular UI → Spring Boot API → MySQL with shared dependencies (Admin-API, Identity-API, Common-API)
• Frontend ↔ Backend port mapping table — lets contributors know which UI talks to which API and on which ports
• Beneficiary data flow — 8-step sequence showing how a patient record moves through registration → nurse → doctor → pharmacy
• New Contributor Journey — 5-step guide with a prerequisites table (Java 17, Maven 3.8+, Node.js 18+, Angular CLI, Docker, MySQL 8.0+, Redis 6+)
• Service startup order — MySQL → Redis → Admin-API → Identity-API → Common-API → target API → target UI
• Common local setup issues — troubleshooting table covering the top errors new contributors hit (referencing existing issues [HelpLine104 API]: Local Setup of the api is not working #122 and [HelpLine 104 UI]: Helpline104 UI Local Setup is not Working #123)

Why this matters

• Reduces time-to-first-PR for new contributors
• Prevents redundant "how do I set up locally?" issues
• Makes the AMRIT monorepo README useful as a navigation hub, not just a link dump
• Follows the pattern established by large OSS projects (Django, Kubernetes) where the root README serves as an onboarding entry point

Test plan

• README renders correctly on GitHub (check table alignment, code blocks, headers)
• All port numbers in the mapping table match values in the individual repo READMEs
• Service startup order is consistent with docker-compose.yml and existing setup guides

🤖 Generated with Claude Code

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