feat(content): add Spring Boot 4.0.5 ecosystem docs (Java)

[giancarmine07]

Summary

Adds curated documentation for the Spring Boot 4.0.5 (CURRENT GA) ecosystem, covering 20 packages targeting Java developers.

All docs are based on the official Spring Boot 4.0.x reference documentation at https://docs.spring.io/spring-boot/ and verified against the 4.0.5 system requirements (Java 17+, Spring Framework 7.0.6+, Jakarta EE 11).

Packages covered

Package	What it covers
web	REST controllers, ProblemDetail (RFC 9457), HTTP Service Clients
data-jpa	Entities, repositories, Flyway migrations, auditing, Hibernate
data-mongodb	Documents, MongoRepository, aggregations, MongoTemplate
data-redis	RedisTemplate, caching, pub/sub, JSON serialization
security	SecurityFilterChain, JWT auth, OAuth2 resource server, method security
testing	MockMvc, RestTestClient (new in 4.0), @DataJpaTest, Testcontainers
actuator	Health checks, metrics, Prometheus, custom endpoints
observability	Micrometer, OpenTelemetry tracing, structured logging (ECS/Logstash)
caching	Caffeine, Redis, @Cacheable/@CacheEvict
messaging	Apache Kafka, RabbitMQ (AMQP), dead letter queues
scheduling	@scheduled, @async, virtual threads, cron expressions
mail	SMTP, HTML emails, Thymeleaf templates, attachments
logging	Logback config, structured logging, MDC, runtime level changes
websocket	STOMP, SockJS, real-time messaging, security integration
webclient	WebClient reactive HTTP, streaming, error handling
validation	Bean Validation, custom validators, groups, @ConfigurationProperties
batch	Job/step config, chunk processing, readers/writers, scheduling
packaging	Docker images, GraalVM native, AOT, Kubernetes probes
docker-compose	Dev-time container management, @Serviceconnection
cloud	Eureka, Config Server, Gateway, Resilience4j, OpenFeign

Key focus areas for AI agents

Each doc highlights:

• Do NOT use sections — deprecated APIs, wrong imports (javax vs jakarta, Jackson 2 vs 3)
• Common Mistakes — the errors AI agents make most often
• Working code examples — copy-paste ready, verified against 4.0.5
• Official source links — for deeper reference

Spring Boot 4.0 breaking changes documented

• Jackson 3 (tools.jackson.* replaces com.fasterxml.jackson.*)
• Undertow removed (Tomcat/Jetty only)
• @JsonComponent → @JacksonComponent
• spring-boot-starter-aop → spring-boot-starter-aspectj
• @MockBean → @MockitoBean
• All 3.x deprecated APIs removed
• Migration path: 3.x → 3.5.x → 4.0.x

Sources

• Official docs: https://docs.spring.io/spring-boot/
• System requirements: https://docs.spring.io/spring-boot/system-requirements.html
• Release notes: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-4.0-Release-Notes
• Migration guide: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-4.0-Migration-Guide