Improve extensions.doc format with admonitions (#5055)

More information can be seen in the Docusaurus doc:
https://docusaurus.io/docs/next/markdown-features/admonitions

website/docs/introduction/extensions.md

@@ -13,9 +13,13 @@ The associated **code samples** to this guide can be found in the package [detek
 _detekt_ uses the `ServiceLoader` pattern to collect all instances of `RuleSetProvider` interfaces. 
 So it is possible to define rules/rule sets and enhance _detekt_ with your own flavor. 
 
-Attention: You need a `resources/META-INF/services/io.gitlab.arturbosch.detekt.api.RuleSetProvider` file which 
+:::caution Attention
+
+You need a `resources/META-INF/services/io.gitlab.arturbosch.detekt.api.RuleSetProvider` file which 
 has as content the fully qualified name of your `RuleSetProvider` e.g. `io.gitlab.arturbosch.detekt.sample.extensions.SampleProvider`.
 
+:::
+
 You can use our [GitHub template](https://github.com/detekt/detekt-custom-rule-template) to have a basic scaffolding to
 develop your own custom rules. Another option is to clone the provided [detekt/detekt-sample-extensions](https://github.com/detekt/detekt/tree/main/detekt-sample-extensions) project.
 
@@ -104,9 +108,13 @@ By specifying the rule set and rule ids, _detekt_ will use the sub configuration
 
 ```val threshold = valueOrDefault("threshold", THRESHOLD)```
 
-Note: As of version 1.2.0 detekt now verifies if all configured properties actually exist in a configuration created by `--generate-config`.
+:::note
+
+As of version 1.2.0 detekt now verifies if all configured properties actually exist in a configuration created by `--generate-config`.
 This means that by default detekt does not know about your new properties.
-Therefore we need to mention them in the configuration under `config>excludes`:
+Therefore we need to mention them in the configuration under `config>excludes`.
+
+:::
 
 ```yaml
 config:
@@ -156,6 +164,7 @@ class NumberOfLoopsProcessor : FileProcessListener {
 	}
 }
 ```
+
 To let detekt know about the new processor, we specify a `resources/META-INF/services/io.gitlab.arturbosch.detekt.api.FileProcessListener` file 
 with the full qualify name of our processor as the content: `io.gitlab.arturbosch.detekt.sample.extensions.processors.NumberOfLoopsProcessor`.
 
@@ -243,4 +252,4 @@ you created a pure kotlin module which has no Android dependencies. `apply plugi
 
 In detekt you can write custom rules which can manipulate your code base.
 For this a cli flag `--auto-correct` and the gradle plugin property `autoCorrect` exists.
-Only write auto correcting code within the `Rule#withAutoCorrect()`-function.
+Only write auto correcting code within the `Rule#withAutoCorrect()`-function.