[DOC] quarkus-extension.yaml format and fields not explined in Quarkus web-docs

[alexeysharandin]

Describe the bug

Hi everyone,
I found reference to quarkus-extension.yaml only here https://quarkus.io/guides/writing-extensions but without any explanations whats fields can be added in yaml and what these fields doing.

based on review of yaml's from extentions and java code I found

group-id: 
artifact-id:
metadata:
  unlisted: 
  config:
    - "quarkus.***"

Maybe something else exists but not required for me at this stage.

Is possible to improve docs? It's not require a lot of time.

Expected behavior

No response

Actual behavior

No response

How to Reproduce?

No response

Output of uname -a or ver

No response

Output of java -version

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GraalVM version (if different from Java)

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Quarkus version or git rev

No response

Build tool (ie. output of mvnw --version or gradlew --version)

No response

Additional information

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[geoand]

cc @aloubyansky @gastaldi