Update images from snippet changes.

[jonmeow]

Note, to see the results, here's a README link (note this is for the 654a06e commit)

Enumerating changes:

• Finish the work started in #3735 by updating the images.
  • Reverts changes to vector initialization style made in #3735, instead preferring https://abseil.io/tips/88 style
• Switch array syntax to match #syntax discussion
• Some nuanced highlighting changes to try matching what I'm seeing in GitHub.
  • The original intent was to match GitHub's highlighting, at least for C++; Carbon highlighting is expected to be poor because it's not a supported language.