bn: Add Bengali file with ~150 translations

[raselmandol]

Added Bengali translation (bn.po) file

First part of #653.

[google-cla]

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[mgeisler]

Awesome, thanks @raselmandol! The rendered course can now be found as a build artifact on the bottom of this page: https://github.com/google/comprehensive-rust/actions/runs/5058634906

[mirwasi]

Thanks @raselmandol for your contribution! I'm approving your pull request.

[mirwasi]

@raselmandol: The Contributor License Agreement (CLA) check is failing. Could you please sign the contributor CLA as per https://github.com/google/comprehensive-rust/blob/main/CONTRIBUTING.md#contributor-license-agreement?

[raselmandol]

@mirwasi , I have signed the contributor CLA now, could you please check again?

[mirwasi]

All good now, thanks!