common style for figures, diagrams, images

[dckc]

We may want to consider a common design patter for all of our dev diagrams.
(based on a comment by @Jovonni)

• some screenshots have borders; some do not
• https://docs.agoric.com/guides/zoe/contract-walkthru.html uses mermaid diagrams
• https://docs.agoric.com/guides/js-programming/hardened-js.html#object-capabilities-ocaps has an SVG taken from a slide
  • likewise https://docs.agoric.com/guides/js-programming/far.html#far-remotable-and-marshaling

[dckc]

Mermaid Class Diagrams

for example:

• chore: replace ERTP diagram with class diagram (WIP) #1140

for ref:

• UML Relationships Types: Association, Dependency, Generalization
• Class diagrams | Mermaid