added model resources for xlm-roberta
# What does this PR do?

Fixes [20055](https://github.com/huggingface/transformers/issues/20055)

- I created a link to task guides for casual language modeling and text classification. I think they are useful and applicable but not directly related to the xlm-roberta model class per say. 
-  For casual language modeling, should I write it under "text-generation" pipeline tag or create a subheader like multiple choice?
- I've checked notebooks from the community but so far none of them do a tutorial on xlm-roberta. Hopefully, there'll be one soon!
- I've also found a few blog posts related to roberta but not xlm-roberta. should we include them? cus they are technically the same architecture just that one is multi-lingual and the other is not


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## Before submitting
- [x] This PR improves the docs of xlm-roberta by adding common and most used resources
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [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).

@stevhliu please check the work and let me know if I need to do any changes. thanks
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