[i18n] Adding Bengali Structure

[fellyph]

Motivation for the change, related issues

This pull request adds Bengali (bn) translations for key user interface elements and documentation in the WordPress Playground project. This PR will serve as a base to @devmuhib009 working with WPCredits on the Translations to Bengali. The changes include translation files for navigation, footer, sidebar, and a comprehensive coding standards guide aimed at Bengali-speaking users and contributors.

Localization of UI Elements:

• Added Bengali translations for the navigation bar, including labels for Documentation, Blueprints, Developers, and API Reference in navbar.json.
• Added Bengali translations for the footer, covering section titles, link labels, copyright, and logo alt text in footer.json.
• Added Bengali translations for sidebar categories and labels for Documentation, Blueprints, Developers, and related sections in current.json.

Documentation:

• Added a Bengali version of the coding standards guide, detailing error messaging, public API principles, and blueprint usage guidelines in main/contributing/coding-standards.md.

[fellyph]

@devmuhib009, can you create a GitHub issue to track the translations, like this one #2320