Architecture diagram

[TomNicholas]

@maxrjones quite rightfully pointed out that the current and intended relationship between kerchunk, virtualizarr and zarr is pretty complicated, and changing rapidly. He suggested that a visual representation would make it a lot easier for users / other developers to understand.

We could use the c4 model for this, and maybe use mermaid to render it on github in a way that can be easily modifed through PRs. (see also this example)