Define and document API for extensions developers

[rsvoboda]

Define and document API for extensions developers

Possible deliverables:

• JavaDoc
  • artifact in maven central
  • artifact attached to https://github.com/quarkusio/quarkus/releases
• Enhanced https://quarkus.io/guides/extension-authors-guide
  • mention suggested APIs and existence of the JavaDoc

This task should also cover Jandex documentation as it's the key component.

fyi @manovotn

[manovotn]

From user standpoint, I think there is quite some gap between knowing the principles of how to write an extension (which we document well in https://quarkus.io/guides/extension-authors-guide) and writing an actual extension.

You are supposed to use build items as entry points of processing but user hardly knows which entry points are there in the first place and what they actually mean. It might be useful to have some basic ones coming from quarkus and arc highlighted/explained as most extension will work on those.

On top of that there is Jandex which might prove to be another obstacle for new users. I know there is documentation but it's outside Quarkus - maybe we could incorporate some examples with it into Quarkus (and docs)?