Add guidance on creating a parallel did:web for did:webvh services and alsoKnownAs

[swcurran]

Extend the guidance in the section on creating a parallel did:web to a did:webvh to include:

• Add the implicit did:webvh services that get added to each did:webvh if not specified.
• Add to the did:webvh DID Doc the alsoKnownAs for the did:web.