Add Portuguese (Brazil) translation files

[BrunoSFreschi]

This pull request adds a new translation guide for Brazilian Portuguese contributors, providing clear instructions on how to translate Rust by Example using the mdbook-i18n-helpers workflow. The guide covers tool installation, translation file management, and how to add new language entries in related configuration files.

New translation workflow documentation:

• Added a comprehensive TRANSLATING_PT.md file outlining the translation process for Rust by Example, including tool requirements, step-by-step commands for creating and updating translations, and instructions for tracking translation progress.
• Provided guidance on editing translation resources and compiling the translated book using the appropriate environment variables and commands.

Integration with project configuration:

• Included detailed instructions for adding new language entries to .github/workflows/rbe.yml, theme/head.hbs, and src/bootstrap/src/core/build_steps/doc.rs to ensure proper integration of new translations into the build and deployment process.

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