Add multi-language support using mdbook-i18n-helpers

[Mameta29]

To add multi-language support to the project using mdbook-i18n-helpers. This will enable us to create translations of our documentation efficiently.

Tasks:

• Install mdbook-i18n-helpers
• Update book.toml to include gettext preprocessor
• Create initial PO template file
• Update build and serve scripts to support multiple languages
• Update README with instructions for translators

This will lay the groundwork for creating translations, starting with Japanese.