[FEATURE] Illustrations on the developer doc

[smiled0g]

We need illustrations on our Developer Doc.

Motivation

Right now it's dull and not really tasteful.

Suggested Implementation

Let's go through this check list

• @sorawit list out all the specs for illustrations needed
• @prin-r draft the illustrations and align with @sorawit on the result
• @smiled0g turns the drafts into full version
• @sorawit then add all illustrations to the doc