Looking into shaders, jme3 have .j3md files. The shader and vertex shaders are well documented elsewhere (as they aren't limited to jme3). But the .j3md files have issues in documentation.
In this tutorial thingy it mentions this post.
There is no longer an attachment (visible one at least) but it links to this which as you can see, is a 404 error.
A similar issue (documentation pointed wrong) was presented in this post from 2015. In a reply, there was a request to update this post (directed to the author of the post). As you can see from that link as well, there is a 404 error.
So where do you go to learn how to use .j3md files these days?