Promote and document ZIM-friendly apps

The latest in ZIM-enabled softare is [kage](https://github.com/tamnd/kage), but there are a lot more out there - and the trend is up.

This is good, but we need to make sure people know they can and should reach out so we make a bit of noise around them - particularly if their project is FOSS. The shout-out can go through our social (mastodon + newsletter), but where should be keep an directory (and make sure the listed projects are active)?

Pinging @tamnd to have first-hand feedback of how they would like things to go to make it easier for them.

Start by reading the issue and its comment thread, including the feedback requested from the kage maintainer. Clarify where the directory should live, how projects are checked for activity, and how Mastodon and newsletter promotion should work; done means those decisions are documented and the agreed directory and outreach process are in place.

Written by the indexing model from the issue text.