Extension author guide: explain how to deal with `META-INF/services` files

[FroMage]

https://quarkus.io/guides/extension-authors-guide#register-resources-for-use-in-native-image led me to believe I only needed to register the file, but actually I need to call ServiceUtil to register the contents of the file for reflection, at the very least.

[gsmet]

@FroMage care to provide a PR? It would be nice to close all these issues that are fixable in one hour instead of having them piling up.

Thanks.

[FroMage]

It would be nice to close all these issues that are fixable in one hour instead of having them piling up.

Famous Last Words: it's been days and no resolution in sight :(

[FroMage]

Follow up at #5582