docs: update smart contracts async flow diagram

[VolodymyrBg]

Proposed changes

1. Updated the smart_contracts_async_flow.svg diagram in the custom contracts tutorial to:

• Add the Blockchain Connector component to accurately represent the architecture
• Improve the flow visualization with more detailed steps
• Add color coding for better visual distinction between components
• Include a note about event subscription options
• Make the diagram more comprehensive and modern

2. Removed the TODO comment from the custom_contracts/index.md file

Fixes # <issue_number_here>

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Types of changes

• Bug fix
• New feature added
• Documentation Update

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Please make sure to follow these points

• I have read the contributing guidelines.
• I have performed a self-review of my own code or work.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generates no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• My changes have sufficient code coverage (unit, integration, e2e tests).

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Screenshots (If Applicable)

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Other Information

The previous diagram was simplified and didn't accurately represent the role of the Blockchain Connector or the complete flow of events. This update provides a more accurate and detailed representation of how custom contract interactions work in FireFly, which will help developers better understand the asynchronous nature of blockchain transactions.

[EnriqueL8]

Thanks @VolodymyrBg this is super useful, it seems that the SVG is cut and I can't see the store events flow and emit events to the App, could you please add that?

[VolodymyrBg]

@EnriqueL8 LIKE THIS?

[EnriqueL8]

Thanks , looking better! A few changes needed please

[VolodymyrBg]

@EnriqueL8 Corrected. I can squash commits if it's necessary, cause too many of them. Sorry for mess