[EPIC] Consistent diagrams

## Expected Behavior

Diagrams should be available for all major Knative concepts and workflows, and should be consistent in the use of color, icons, and creation tools.

## Actual Behavior

Many diagrams don't exist; those that do are inconsistent and often messy or out of date. There are no guidelines for creating diagrams.

## Tasks (to be split into issues)

- [x] #5502
- [x] #5503
- [x] #5504
- [x] #5505

Review the completed task issues #5502, #5503, #5504, and #5505 to determine what remains from this epic. Compare the existing diagrams with the stated consistency goals for colors, icons, creation tools, and coverage of major Knative concepts and workflows. Done requires the remaining gaps to be identified and diagram guidelines to be established.

Written by the indexing model from the issue text.