added diagram to explain webhooks

doc update click links for preview:

Added a small section to the main readme:
https://repo1.dso.mil/platform-one/big-bang/bigbang/-/tree/BBOI-653_diagram-kube-apiserver-webhooks/docs/understanding_bigbang#note-about-snippets-of-architecture-diagrams-in-this-folder

Added an architecture diagram about webhooks to help new users understand them
https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/BBOI-653_diagram-kube-apiserver-webhooks/docs/understanding_bigbang/kube-apiserver_webhooks_diagram.md