Documentation links to examples are broken
### Describe the bug

The links at the bottom of [add_dataset](https://huggingface.co/docs/datasets/v1.2.1/add_dataset.html) to examples of specific datasets are all broken, for example

- text classification: [ag_news](https://github.com/huggingface/datasets/blob/master/datasets/ag_news/ag_news.py) (original data are in csv files)

### Steps to reproduce the bug

Click on links to examples from latest documentation

### Expected behavior

Links should be up to date - it might be more stable to link to https://huggingface.co/datasets/ag_news/blob/main/ag_news.py

### Environment info

dataset v1.2.1