Updated vocabulary and diagram

[iherman]

Now that the DI spec itself is ready for CR, this PR syncs up the vocabulary, including the diagram, with the spec. The changes are:

• The vocabulary (and the diagram) includes the digestMultibase term, with the caveat in the spec (whereby the term is at risk, and may change for digestSRI) copied into the vocabulary as well. Note that this is the only hard, content change in the vocabulary.
• The diagram changes discussed in #175 are now moved to the “real” branch
• There is now an alt text for the diagram

As usual, there is a preview of the results

[msporny]

Editorial, multiple reviews, changes requested and made, no objections, merging.