Add the kafka-basics module

kafka-basics/docs/01-the-on-ramp.md

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+[Module README](../README.md) · [Producing →](02-producing.md)
+
+# 1. The on-ramp, and the dependency that is not the one you remember
+
+Under Spring Boot 3 you added `org.springframework.kafka:spring-kafka` and got a
+`KafkaTemplate`. Under Boot 4 you get this:
+
+```
+No qualifying bean of type 'org.springframework.kafka.core.KafkaTemplate<java.lang.String,
+com.ankurm.kafkabasics.OrderEvent>' available
+```
+
+Boot 4 split the auto-configurations out of `spring-boot-autoconfigure` into per-technology
+modules. Kafka's now lives in `spring-boot-kafka`, package
+`org.springframework.boot.kafka.autoconfigure`, and a bare `spring-kafka` dependency does not
+bring it. The application compiles, the context starts, and there is simply no template.
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-kafka</artifactId>
+</dependency>
+```
+
+That is the fix, and the same shape applies elsewhere: `spring-boot-starter-amqp` for RabbitMQ,
+`spring-boot-starter-restclient` for `RestClient.Builder`. The rule of thumb for Boot 4 is that
+if you are depending on a library directly rather than through a Boot starter, you are probably
+missing its auto-configuration.
+
+## What the starter actually gives you
+
+| Bean | Comes from | Notes |
+|---|---|---|
+| `KafkaTemplate<?, ?>` | `KafkaAutoConfiguration` | typed by your injection point |
+| `ProducerFactory` / `ConsumerFactory` | `KafkaAutoConfiguration` | built from `spring.kafka.*` |
+| `KafkaListenerContainerFactory` | `KafkaAnnotationDrivenConfiguration` | what `@KafkaListener` binds to |
+| `KafkaAdmin` | `KafkaAutoConfiguration` | creates `NewTopic` beans at startup |
+
+`KafkaAdmin` is worth knowing about early: declare a `NewTopic` bean and Boot creates the topic
+on startup with the partition count and replication factor you asked for. It will **not** change
+an existing topic's partition count, so a `NewTopic` bean that disagrees with the cluster is
+silently ignored rather than applied.
+
+## Versions
+
+Boot 4.1.1 manages Spring Kafka **4.1.1** and kafka-clients **4.2.1**. Note the second one:
+Maven Central has kafka-clients 4.3.1, and overriding the managed version to reach it is the
+kind of change that works until it does not. Let the BOM decide.
+
+[Producing &rarr;](02-producing.md)