Dataframe HTML tags showing up in exported README

Related to https://github.com/fastai/nbdev/issues/474

Rendered readme on Github: https://github.com/rxavier/poniard/blob/experiment/nbdev/README.md
index.ipynb: https://github.com/rxavier/poniard/blob/experiment/nbdev/nbs/index.ipynb

Basically, "raw output" dataframes (for example, when a method simply returns a dataframe, like `get_results` in this example) get this added before the actual table in markdown:

```
.dataframe tbody tr th:only-of-type { vertical-align: middle; }
.dataframe tbody tr th {
vertical-align: top;
}

.dataframe thead th {
text-align: right;
}

```

Meanwhile, dataframes rendered with `display(HTML(df.to_html()))` (which shows after `setup` in this example) display fine.

Quarto HTML docs render any of these fine.

Edit: I implemented an ugly temporary fix by using directives and ipython's `display`:

```python
#| output: false

obj.method_that_outputs_a_df()
```

```python
#| echo: false

from ipython import display, HTML

df = obj.method_that_outputs_a_df()
display(HTML(df.to_html()))
```

Start with nbs/index.ipynb and the exported README linked in the issue, then reproduce a raw dataframe return such as get_results and compare it with display(HTML(df.to_html())). The fix is complete when exported README markdown renders the dataframe without the unwanted style tags, while the table remains visible.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.