Add documentation and sample on how to start with Dapr using Spring Boot

1. Update the [Java SDK docs](https://docs.dapr.io/developing-applications/sdks/java/) to include a page (suggested name: _Frameworks_) on how to start with Dapr using Spring Boot. Note that there will also be content for Quarkus (Issue: #1037).
Once this information is there, we can link to this page from the [SDK overview](https://docs.dapr.io/developing-applications/sdks/).

2. Add a minimal code sample for Dapr + Spring Boot in the [examples](https://github.com/dapr/java-sdk/tree/master/examples) folder.

Related docs issue: https://github.com/dapr/docs/pull/4095