Portal: Add SwapWidget in bridge/swap page (#8072)

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## PR-Codex overview
This PR focuses on updating the documentation for the `SwapWidget` component, simplifying the swap interface example, and enhancing user guidance on configuring default token selections.

### Detailed summary
- Added `DocImage` component to display `swap-dark.png`.
- Updated the swap interface example to use `SwapWidget`.
- Simplified the example function to `Example`.
- Provided instructions for configuring default token selections.
- Included code snippets for various default configurations.
- Removed the detailed implementation of the previous swap interface.

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Author: MananTank