DynamoDB on-demand pricing is stale (2x too high) - fix in #26

The DynamoDB on-demand throughput rates in this repo predate the **November 1, 2024**
price cut, which halved them ([announcement](https://aws.amazon.com/blogs/database/new-amazon-dynamodb-lowers-pricing-for-on-demand-throughput-and-global-tables/)). The figures currently overstate on-demand
throughput cost by 2x.

| | Listed here | Current |
|---|---|---|
| Write request units | $1.25 /M | **$0.625 /M** |
| Read request units (strongly consistent) | $0.25 /M | **$0.125 /M** |

Authoritative source, versioned and machine-readable:

```
https://pricing.us-east-1.amazonaws.com/offers/v1.0/aws/AmazonDynamoDB/current/region_index.json
```

**Affected:**

- [`chatbot/cost_estimate_widget.py`](https://github.com/aws-samples/sample-devgenius-aws-solution-builder/blob/main/chatbot/cost_estimate_widget.py)

**Fix:** #26

Storage (`$0.25/GB-month`) and provisioned rates were not affected by the cut and are unchanged in
that PR.

Comienza en chatbot/cost_estimate_widget.py y compara los valores de rendimiento bajo demanda de DynamoDB con la fuente de precios versionada de AWS en la URL del issue. Actualiza las tarifas de escritura y de lecturas fuertemente consistentes a los valores actuales y, después, verifica que las tarifas de almacenamiento y aprovisionadas no hayan cambiado.

Escrito por el modelo de indexación a partir del texto del issue.