Add Bengali translation for index.md

[talha937]

Starts the Bengali translation of the huggingface_hub docs.

Translates:

• docs/source/bn/index.md
• docs/source/bn/_toctree.yml

Also registers bn in the CI language list so the translated docs build correctly.

Contributes to #4680

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Note

Low Risk
Documentation and workflow language-list changes only; no runtime library or security impact.

Overview
Introduces Bengali (bn) as a documentation locale for huggingface_hub, starting with the landing page and navigation shell.

Adds docs/source/bn/index.md (Bengali translation of the Hub client library intro, quick-start pointer, and section cards for guides/reference/concepts) and docs/source/bn/_toctree.yml with a “Get started” section linking to the home page. CI is updated in build_documentation.yaml and build_pr_documentation.yaml to include bn in the languages list so main and PR doc builds publish the new locale alongside existing ones.

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[talha937]

@Wauplin @stevhliu could you please review this when you get a chance? This is my first contribution to huggingface_hub 🙂