Update examples and diagrams in section 5.

[peacekeeper]

This updates the examples and diagrams in section 5 in a few ways:

• Use of SVG instead of PNG (partially addresses Use SVG images instead of PNG #128)
• Rename "Decentralized Identifier Registry" -> "Verifiable Data Registry"
• Fix DID parameter name relative-ref -> relativeRef
• Use Multikey instead of Ed25519VerificationKey2018
• Align data shown in example listings with data shown in diagrams
• Two separate diagrams instead of one
• Use 1.1 DID JSON-LD context

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[peacekeeper]

Sorry, accidentally created this PR from a wrong branch. Use this instead: #142