Refactor/jep info model into flat structure and extend

[ab-schneider]

Tasks done:

• Flatten JSON structure (remove unnecessary info nesting)
• Update JEPInfo.java accordingly
• Add new parameters: jdk, link, code
• Update all existing JSON files to the new format
• Update contribution guide to align with the new structure

Few remarks:

• I used Cursor to prefill the link and code parameters, but then manually checked every JEP, and correct several mistakes. So you probably could skip review of resources (or do random check) otherwise it will take you few hours, as it took me.
• Order of JEPs not always was respected, I restore it.
• There we 2 or 3 JEPs missing from JSON model. I added them
• Fixed spelling mistakes in CONTRIBUTING.md

Closes #302

[AloisSeckar]

The important changes look fine. If there is something a little off, we will find and fix eventually.

Thank you very much @ab-schneider for this really valuable and hard work that will move this project forward 🫡