AI Gateway: Update Mermaid docs to reflect 2.x abstractions

## Jobs to be done (optional)

Commented out diagrams in the following pages:
- app/_ai_gateway_policies/ai-azure-content-safety/index.md
- app/_ai_gateway_policies/ai-custom-guardrail/index.md

## Definition of done

All Mermaid diagrams in AIGW docs should be verified against the new architecture

## Information

See this conversation: https://kongstrong.slack.com/archives/G012HLJ4GJJ/p1787738558402519

## Size

- M (Medium) Example: writing a UI how to for a straightforward product, adding a plugin example