Add an example of simple VC

In trying to help people understand did:onion (and get some funding from patrons) I really need to also demonstrate a simple signed VC.

What I'd like to have is a simple self-signed creator claim from the Creative Works schema https://schema.org/CreativeWork along the lines of:

```
issuer: did:onion:XXXX
subject: XXXX.onion/filename.git
creator: did:onion:XXXX
````

Just sticking within the limitations of CreativeWorks schema I think we can get a lot of the things I want for the signed code use case.

Is there any way you could add that simple example in your default javascript demo for did:onion? And include an example in the appendix of this did:onion spec?

Once we have a good example we'll parallel it in the rust cli app.