Updated the Model docs - for the MARIAN model
Author: emanrissha
Pull request: yes

This update improves the Marian model card to follow the Hugging Face standardized model card format. The changes include:

- Added a clear description of MarianMT, its architecture, and how it differs from other models.
- Provided usage examples for Pipeline and AutoModel.
- Added a quantization example for optimizing model inference.
- Included instructions and examples for multilingual translation with language codes.
- Added an Attention Mask Visualizer example.
- Added a Resources section with relevant links to papers, the Marian framework, language codes, tokenizer guides, and quantization documentation.
- Fixed formatting issues in the code blocks for correct rendering.

This update improves the readability, usability, and consistency of the Marian model documentation for users.

# What does this PR do?

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## Before submitting
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Comments:

hey @stevhliu can you help me with this one? this is my first time making a PR here 🤗. Hit me up if you need me to make any further updates in the docs.




@stevhliu I've made a pass on your feedback. Let me know if you see some new things to change/improve.



@stevhliu I made the changes you requested — thank you for your guidance! This is my first time creating a doc, so I really appreciate your help.



The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39138). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.