First, there’s a dedicated forum for shaders, so you might have more luck over there.
Second, this could potentially be fairly complicated, depending on how flexible you want the shader to be. If it’s mainly just lightening pixels near the edge of a mesh, then that’s simpler. But things could get more tricky if you want transparency and cutout, and how you want to deal with complex meshes of multiple triangles. For example, what if you had several triangles connected to each other? Would only the outer edges be faded? Or do you never do that?
For the simplest case, the complex task is determining the distance of any given pixel from an edge. The following links discuss edge detection approaches:
I probably wouldn’t pick this as my first attempt to learn shaders, but it might be straightforward enough. You’ll probably need to go through Unity’s shader tutorials first to have any idea what’s going on in the shaders, though. It’s a whole new language to understand and very different conventions: