docs: add back the title column from demo dataset

[fhk]

As per #1

I'd also like to add the following doc for future reference but couldn't decide which file it should live in.

## Example data

The data is created as part of the [GitHub Actions](https://github.com/apple/embedding-atlas/blob/main/.github/workflows/ci.yml).

Which calls the following script

[embedding-atlas/blob/main/packages/docs/generate_demo_data.py](https://github.com/apple/embedding-atlas/blob/main/packages/docs/generate_demo_data.py)

[donghaoren]

Thanks! We don't have a a README for how to build the docs yet, I'll take a note and add your suggestion later.