The RefApp list is static. It (and all the lists) are defined in this section of the add-ons-to-index file.
I think we can automatically determine the refapp release lists based on the openmrs-distro.properties files in the released refapp versions. It would make sense to determine this and some other lists (e.g. top-10-downloads) dynamically, so I'll add that to the backlog.
I was holding off of putting "how to add your add-on" instructions until we actually replace modulus. But I guess we can put it up sooner. (There is a placeholder https://addons-stg.openmrs.org/#/about page, but I guess the link to this got lost in some of the UI enhancements. )