Add multi-language support using mdbook-i18n-helpers

[Mameta29]

This PR adds multi-language support to the project using mdbook-i18n-helpers. This lays the groundwork for creating translations of the documentation, starting with Japanese.

Changes implemented:

• Installed mdbook-i18n-helpers
• Updated book.toml to include gettext preprocessor
• Created initial PO template file
• Updated README with instructions for translators

To test:

1. Install mdbook-i18n-helpers: cargo install mdbook-i18n-helpers
2. Generate POT file: MDBOOK_OUTPUT='{"xgettext": {}}' mdbook build -d po
3. Create a PO file for a new language (e.g., Japanese): msginit -i po/messages.pot -l ja -o po/ja.po
4. Add some translations to po/ja.po
5. Build the Japanese version: MDBOOK_BOOK__LANGUAGE=ja mdbook build -d book/ja