🌐[i18n-bn] Introduce Bengali version of Transformers documentation

[ankitdutta428]

What does this PR do?

This PR adds a new Bengali localization section to the documentation of the Transformers library.

• Introduces a new folder: docs/source/bn/
• Adds a localized file: index.md providing an introduction to the Transformers library in Bengali
• Adds a new file: i18n/README_bn.md to guide Bengali-speaking users with general information about the library

This is part of an effort to make the Hugging Face documentation more accessible to Bengali-speaking developers, researchers, and learners.

Fixes: #39705

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum? 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, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Tagging @stevhliu for documentation-related review.
Any contributor interested in localization or multilingual documentation is welcome to review as well.

[stevhliu]

Thanks for your interest in translating the docs! It looks like you may be translating an older version of index.md and README.md. This should be updated to match the latest version on main. Also, you'll need to find a Bengali reader who can review your translation as I cannot read it!

You also need to include the Bengali language code here and to build the docs in your language!

[ankitdutta428]

Hi I have started to work on the newer versions and will translate them in a few days. I will also add the bengali code in the documentation as instructed. As for the bengali reviewer, do they need to satisfy some requirements before they can satisfy the requirements. I currently do not have any bengali reviewers here on github so if could suggest what to do in this case it would have been a great help. Thank you.

[stevhliu]

There aren't any specific requirements for the reviewer as long as they can understand the translated content and that it makes sense. I would ask on the Hub or Discord to see if anyone is interested in helping out! 🤗

[ankitdutta428]

Okay thank you, that will be a great help. Also can I ask some of the people I know who also know Bengali to review my translated documentation here?