add greek translation to index
# What does this PR do?

Fixes #19788

## 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),


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19789). All of your documentation changes will be reflected on that endpoint.<|||||>Yes, Greek under ISO 639-1 would be `el`, which is what we use. `gre` corresponds to 639-2. 

(source: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes)<|||||>Hello! I am really sorry for the mistake I will fix it shortly!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Sorry missed the change! It looks like there is a problem with CircleCI (tests are not run). Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-)?

Could you also add the Greek language code [here](https://github.com/huggingface/transformers/blob/11f3ec7224c83c9e5c379a774b9d3984e68d26fa/.github/workflows/build_documentation.yml#L18) and [there](https://github.com/huggingface/transformers/blob/11f3ec7224c83c9e5c379a774b9d3984e68d26fa/.github/workflows/build_pr_documentation.yml#L17) so that the doc is built in Greek?

let us know if you need any help, thanks!