add an example of a "document gateway"

[b5]

We had a user run the gateway example & immediately ask for a feature only available in documents: being able to change a directory & see changes reflected on the gateway.

We might want to consider extending iroh-gateway to support joining a document for a single sync to list keys, then drop down to the existing iroh-gateway for blob viewing. We could start simple & only accept a read ticket, then delegate all requests for content to the node addr in the ticket

[rklaehn]

The iroh gateway example is really an example to use iroh-bytes. So I don't think this will be simple. It does not include an iroh node.

But we could have something of a correspondence between documents and nodes. See the discussion topic for the team meeting. E.g. you could have a function to take a snapshot of a document as a collection.