add zipkin spring documentation

[zeitlinger]

What needs to be changed?

zipkin spring documentation is missing.

It used to be here: https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/71b3e3e027f78a2a44007129a4bad32a641d8103/instrumentation/spring/spring-boot-autoconfigure/README.md#features

This should probably be included in https://github.com/open-telemetry/opentelemetry.io/blob/main/content/en/docs/zero-code/java/spring-boot-starter/other-spring-autoconfig.md