Specify root tree layout

[matheus23]

Currently the public and private WNFS are specified separately. Each has their own root CID.
There's also the root for device exchange keys for shared private data.

All of these roots are usually conveniently packed into some root block, traditionally this was a dag-pb block with a public, private and more links, including a version link to the WNFS version.

It may be useful to specify the exact data format for the root block in the WNFS spec.

That spec would only be a recommendation for anyone who wants to bundle public & private file systems together.