Some mermaid chart code cleanups

What does this MR do and why?

Removes one defunct node from the Diffs app front end chart, which didn't have any connections, so it was floating at the top of the graph.

Also does a small amount of cleanup and - as a result of removing that node - shifts the names of a handful of nodes back one letter.

Screenshots or screen recordings

See the "unknown" node has been removed.

Before	After

How to set up and validate locally

1. Probably just use the links to the docs pages in this MR.
   • current vs. updated

MR acceptance checklist

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