docs: clarify prompt-flow architecture diagram source

[amit-chaubey]

Issue #, if available:
N/A

Description of changes:

• Adds a short clarification in guidance/prompt-flow-orchestration/README.md about architecture diagram presentation.
• Notes that this repository currently uses IDP-Diagram-v3.png for the prompt-flow architecture view.
• Clarifies that the AWS Guidance webpage may present diagram content in a tabbed/multi-page format (Diagram 1/2/3), which can look different from the repo asset layout.

Why this change:

• Prevents confusion for contributors and users who compare the repo README with the AWS Guidance webpage/downloaded diagram presentation.

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[amit-chaubey]

Thanks for reviewing. This PR is docs-only and adds a small clarification in guidance/prompt-flow-orchestration/README.md to explain the diagram presentation difference between the repository (IDP-Diagram-v3.png) and the AWS Solutions page (tabbed/multi-page view), so contributors don’t interpret it as a missing asset.