docs: start Bengali (bn) translation

[alrifatsabbir]

This PR starts the Bengali (bn) translation for Comprehensive Rust.

• Initialized po/bn.po with translations for the summary and introduction.
• Added the translation/bn label to .github/labeler.yml.

Verified the .po file syntax with msgfmt --check.

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[alrifatsabbir]

@googlebot I signed it.

[mgeisler]

Hi @alrifatsabbir, thanks for the PR! I asked Gemini about it and it tells me that it's an improvement over the status-quo! @mirwasi and other Bengali speakers should chime in with more fixes on top of this.