Document how to set an extension image.

docs/src/main/asciidoc/extension-metadata.adoc

@@ -32,14 +32,15 @@ metadata:
   - "reactive"
   status: "stable" <6>
   guide: "https://quarkus.io/guides/resteasy-reactive" <7>
-  codestart: <8>
+  icon-url: "https://quarkus.io/assets/images/about/icon-reactive.svg" <8>
+  codestart: <9>
     name: "resteasy-reactive"
     languages:
       - "java"
       - "kotlin"
       - "scala"
     artifact: "io.quarkus:quarkus-project-core-extension-codestarts"
-  config: <9>
+  config: <10>
   - "quarkus.resteasy-reactive."
 ----
 
@@ -50,8 +51,9 @@ metadata:
 <5> Categories the extension should appear under on https://code.quarkus.io[code.quarkus.io]. This field can be omitted, the extension will still be listed on https://code.quarkus.io[code.quarkus.io] but won't be categorized
 <6> Maturity status that could be `stable`, `preview`, `experimental`. It is up to extension maintainers to evaluate the maturity status and communicate it to the users
 <7> Link to the extension guide or documentation page
-<8> https://quarkus.io/guides/extension-codestart[Codestart] information
-<9> Configuration prefix
+<8> Link to an externally hosted image. This is used in the Quarkus dev tools as the extension icon. Alternatively, if you https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/customizing-your-repositorys-social-media-preview[set the social media preview] on your extensions source code repository, the tools will pick up that image.
+<9> https://quarkus.io/guides/extension-codestart[Codestart] information
+<10> Configuration prefix
 
 And here is the final version of the file included in the runtime JAR augmented with other information collected by the Quarkus Maven plugin: