docs: document all 11 starters; add ARCHITECTURE.md and DEVELOPER.md

[SaumilP]

README:

• Available Starters table now lists all eleven starters (was three), with descriptions, config prefixes, and versions.
• Corrected Maven coordinates to the real group io.github.saumilp.starters (badge, Quick Start snippets).
• Project Structure now reflects the .kts build files, every starter/example module, and the new docs.
• Building Locally uses the dedicated integrationTest task; added a documentation index.

New docs:

• ARCHITECTURE.md — module topology and dependency graph, starter anatomy, auto-configuration registration and conditional strategy, observability, build/test architecture, CI/CD, and the Spring Boot 4.0 baseline.
• DEVELOPER.md — prerequisites, Gradle commands, running integration tests, a step-by-step "add a new starter" guide, coding/testing conventions, Spring Boot 4.0 gotchas, publishing/release, and troubleshooting.

Summary

Changes

• README.md updates

Type of change

• Bug fix
• New feature / starter
• Breaking change
• Documentation update
• Dependency update

Checklist

• Unit tests added or updated
• Integration tests added (if touching infrastructure adapters)
• JavaDoc added for all new public methods
• ./gradlew spotlessCheck passes locally
• ./gradlew javadoc compiles without warnings
• README updated (if adding new configuration properties or starter)
• CHANGELOG.md entry added under [Unreleased]