Added translation of index.mdx to Portuguese Issue #16824
# What does this PR do?

Creates folder pt in docs/source for translating documentation to Portuguese
Currently, only the index.mdx file was translated as of this PR.

Fixes issue #16824

## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [X] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [X] Did you write any new necessary tests?


_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you again @rzimmerdev for the translation of `index.mdx` and for correcting mistakes in `training.mdx`!

@sgugger looks good to me :). If possible, this would be a good addition to the next release.