[EPIC] Consistent diagrams

[abrennan89]

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Many diagrams don't exist; those that do are inconsistent and often messy or out of date.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)

• Decide on a tool for diagrams #5502
• Decide on an icon set for diagrams and create a key for icons #5503
• Create a list of diagrams missing from the docs #5504
• Create new diagrams to fill gaps in documentation #5505

[Cali0707]

/cc @mmejia02 @zainabhusain227 is this something you would be interested in working on?