Pipeline Step Documentation Generator improvementsand also find the
Plugin Health Scoring systemidea interesting. Thank you.
I recommend that you start with https://www.jenkins.io/doc/developer/
More specific to Jelly, you have https://www.jenkins.io/doc/developer/forms/jelly-form-controls/
The how-to's might also help you to get started.
Thank you for your interest. You're very welcome to join and contribute to our Ecosystem. The plugin health score is taking more and more substance and will make it in the final list.
Don't hesitate to join us during the GSoC "Office Hour" where we will discuss and clarify projects. This week we will have the EMEA (Europe/Middle-East/Africa) edition (14:00/2PM UTC). Next week will be the Pacific edition (03:00/3AM UTC). We will be alternating meetings to try to cover the globe conveniently.