[i18n] Add Bengali translation for test.md

[noruzzamans]

What?

Added Bengali (bn) translation for the [test.md]

Why?

To make WordPress Playground documentation accessible to Bengali-speaking users and provide an overview of the platform in their native language.

How?

• Translated all content following the established translation format
• Original English text preserved in HTML comments for reviewer reference
• Technical terms (WordPress, Playground, GitHub, etc.) handled using standardized Bengali script while keeping original terms for clarity
• Markdown structure and links remain unchanged
• Code blocks remain unchanged

[fellyph]

Thanks @noruzzamans !