Add French translation of task_summary and tasks_explained
Author: A-Duss
Pull request: yes

# What does this PR do?

I translated the `task_summary.md` and `tasks_explained.md` files into French. For technical terms, I opted to use the English version where it felt more appropriate. When a French Wikipedia page existed for a term, I used the French equivalent; otherwise, I either kept the English term or included it in parentheses alongside the translation. Please feel free to suggest any changes if something seems unclear or off. Thanks in advance for reviewing!

Following #18322

## Before submitting
- [ ] 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#create-a-pull-request),
      Pull Request section?
- [ ] 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.
- [ ] 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).
- [ ] Did you write any new necessary tests?


## Who can review?
@omarespejel @sgugger @stevhliu





Comments:
I realigned the `toctree` (see [`1b9b2f0`](https://github.com/huggingface/transformers/pull/33407/commits/1b9b2f031924b3feb835917ea62ce4aadc5eb37d)). However, while building the documentation and testing it locally, I noticed that the index page isn't properly linked, resulting in a 404 error when trying to access it:
```bash
NotFound [Error]: Not found: /
    at resolve (C:\Users\antoi\AppData\Local\Temp\tmpcdaq9ka7\kit\node_modules\@sveltejs\kit\src\runtime\server\respond.js:483:13)
    at resolve (C:\Users\antoi\AppData\Local\Temp\tmpcdaq9ka7\kit\node_modules\@sveltejs\kit\src\runtime\server\respond.js:277:5)
    at #options.hooks.handle (C:\Users\antoi\AppData\Local\Temp\tmpcdaq9ka7\kit\node_modules\@sveltejs\kit\src\runtime\server\index.js:49:56)
    at Module.respond (C:\Users\antoi\AppData\Local\Temp\tmpcdaq9ka7\kit\node_modules\@sveltejs\kit\src\runtime\server\respond.js:274:40)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  status: 404
}
```
It does not seem like normal behavior to me but the path referred to in the yml file and the actual file name match.
If anyone has an explanation for that,  would be glad to learn and understand it.


As for French contributors that might be able to give it a quick review:
@younesbelkada @fxmarty @ylacombe @regisss @lhoestq @aymeric-roucher @eustlb @trault14 
If you have a moment to check if anything seems off or sounds unusual, I would greatly appreciate it. 
Thank you! :blush:

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33407). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

**LGTM!** Thanks a lot @A-Duss 🤗