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.

Start in chatbot/cost_estimate_widget.py and compare the DynamoDB on-demand throughput values with the versioned AWS pricing source at the URL in the issue. Update the write and strongly consistent read rates to the current values, then verify that storage and provisioned rates remain unchanged.

Written by the indexing model from the issue text.