* Create chat.rst

* Update pyproject.toml

Hotfix for chat

* Update conf.py

Hotfix for chat.rst

This implementation basically resolves #84.

One implementation is different from the #84:
.add_columns(
  cols=[
    {"name": "sender", "type": "string", "udf": compute_sender},
    ...
  ]
)

If add_columns() uses cols, udf, types as params, it will make this function confusing again. Instead, if users need to specify different udfs for different columns, they should just call add_columns() multiple times for different columns.

…al values,

use field_values instead of field_types as field_values have the actual values, since field_values have the actual key-value pairs, while field_types are just contain fields and their types.

records[0].schema is the schema of the output, which doesn't mean we already populate the schema into record.

This change is based on #101 and #102, please review them first then this change.

1. This is to refactor all demos to use .sem_add_columns or .add_columns, and remove .convert().

2. Remove Schema from demos, except demos using ValidationDataSource and dataset.retrieve() that need schema now. We can refactor these cases later.

…to resolve issues w/specifying schemas for DataSources