docs: document usage of operation customizer and replacing Springwolf beans with +

custom implementations (#80)

Co-authored-by: Timon Back <timonback@users.noreply.github.com>

Author: sam0r040

.gitignore

@@ -3,3 +3,4 @@ node_modules
 .docusaurus
 build/**
 .idea
+/*.iml

docs/configuration/customizing.md

@@ -23,7 +23,7 @@ All default implementations are Spring managed beans, which can be overridden.
 
 ## `AsyncApiCustomizer` - Full AsyncAPI document
 
-By implementing the `AsyncApiCustomizer`, the AsyncAPI document can be modified after Springwolf has done all the scanning and has built the document.
+By implementing the `AsyncApiCustomizer` interface, the AsyncAPI document can be modified after Springwolf has done all the scanning and has built the document.
 It's the final interception point before the document is available to the user.
 
 For example, the title can be adjusted - although this should be done through the configuration:
@@ -38,6 +38,32 @@ public class AsyncApiTitleCustomizer implements AsyncApiCustomizer {
 }
 ```
 
+## `OperationCustomizer` - Operation object
+
+By implementing the `OperationCustomizer` interface, the Operation object can be modified after Springwolf has scanned an
+annotated method and extracted the data.
+
+It is possible to create multiple implementations of `OperationCustomizer`.
+The order of execution can be controlled by the `@Order` annotation of Spring.
+
+An example to conditionally add a custom AsyncAPI tag for Kafka batch listeners is shown bellow:
+
+```java
+@Component
+public class TagCustomizer implements OperationCustomizer {
+
+    @Override
+    public void customize(Operation operation, Method method) {
+        KafkaListener annotation = AnnotationScannerUtil.findAnnotation(KafkaListener.class, method);
+        if (annotation != null && annotation.batch().equals("true")) {
+            Tag tag = new Tag();
+            tag.setName("batch");
+            operation.getTags().add(tag);
+        }
+    }
+}
+```
+
 ## ObjectMapper in `DefaultAsyncApiSerializerService`
 
 The `DefaultAsyncApiSerializerService` is responsible for serializing the AsyncAPI document into a `String` for the Controller.
@@ -49,3 +75,16 @@ Use `DefaultAsyncApiSerializerService#getJsonObjectMapper()` and `DefaultAsyncAp
 All `ChannelScanner` beans are called to scan for channels.
 This interface is helpful when a protocol currently unsupported by Springwolf is used.
 Remember to register all payloads in the `ComponentsService`.
+
+## Customize internal behavior  
+
+Springwolf uses `@ConditionalOnMissingBean` annotations for almost all internal Spring beans.
+If you have a special use-case that requires custom logic,
+you can replace almost every Springwolf bean by adding your custom implementation as a bean to the Spring context.
+
+:::note
+Replacing Springwolf beans with custom implementations is only for experts,
+and we do **not** offer support for using internal API.
+
+Custom implementations may break during updates without notice.
+:::