Add an icon for javadocs

[NotMyFault]

Previously, the plugin used the default help icon, but it doesn't look well in a list view. I replaced it with a Duke sprite. Additionally I added an index.jelly to provide a bit of information in the plugin manager.

@oleg-nenashev (if you're (still) the project maintainer 👀)

Before	After

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[oleg-nenashev]

This is a very good question whether I am a project maintainer. I'm listed as one though, and the pull request looks good to me