Add OCI Image diagram

[abitrolly]

Diagrams are awesome. My attempt at drawing OCI image structure hit the bug that it is impossible to set node order in mermaid mermaid-js/mermaid#3253 Otherwise it looks like this.

flowchart LR
  subgraph image.tar
    direction LR
    subgraph /
      direction LR
      OL["fa:fa-file oci-layout"]
      OX["fa:fa-file index.json"]
      BL["blobs/"]
    end
    sha256/358...
    BL --> sha256/358...
    BL --> sha256/4b0...
    BL --> sha256/796...
  end

mermaid.live for enhancements.

[vbatts]

oh wait. You did this in markdown and it renders like that on Github?
I wonder if that can also export to *.png?

[sudo-bmitch]

We've got some pictures in the img directory. The current method for building with code uses the dot file. There's a makefile entry for applying these.

https://github.com/opencontainers/image-spec/blob/main/Makefile#L98

I'm fine with changing methods to generate pictures from code/text, but we should consolidate on a single method for maintainability.