Self-guided training template

[maglietti]

Summary

Restructure the training project into a "hands-on self-guided training template" that users build step-by-step following modular documentation.

Key changes:

• Break monolithic README into focused training modules in docs/
• Add complete code snippets with JavaDoc comments to each module
• Add "Understanding Entity Classes" section covering schema-first development and POJO design
• Add "Understanding Ignite 3 Data Access APIs" section covering RecordView, KeyValueView, and SQL
• Add sequence diagram for request flow in the architecture reference

What's Changed

Documentation structure:

• docs/01-cluster-setup.md - Docker cluster and data loading
• docs/02-spring-boot-config.md - Dependencies, connections, entity mapping
• docs/03-repositories.md - Spring Data repositories with query derivation and custom SQL
• docs/04-rest-api.md - REST controller
• docs/reference/architecture.md - How components fit together
• docs/training-overview.md - Learning objectives and prerequisites

Template baseline:

• pom.xml starts without Spring dependencies (added in Step 5)
• Application.java starts as bare Spring Boot app (modified in Step 8)
• application.properties starts minimal (configured in Step 7)
• Repository and controller files don't exist (created in Modules 3-4)
• Entity classes (City.java, Country.java) are provided with educational JavaDoc

New instructional content:

• Schema-first development approach explanation
• SQL to Java type mapping reference table
• Spring Data JDBC vs Ignite native annotations comparison
• Ignite 3 Table API overview (RecordView, KeyValueView, SQL with Statement builder)