Add mdbook-i18n-helpers for multi-lingual support

[benjamink]

Summary

• Configures book.toml with the gettext preprocessor and xgettext output backend
• Adds po/messages.pot — the extracted translation template (generated from all source markdown)
• Updates the GitHub Actions deploy workflow to install mdbook-i18n-helpers (with binary caching) and auto-build a translated site for every po/*.po file found, placing each at book/<lang>/

Adding a new language

# Initialize translation file
msginit -i po/messages.pot -l es -o po/es.po

# Edit po/es.po with Poedit or any PO editor, then commit
# The next deploy will automatically publish Spanish at /es/

Test plan

• Verify English build still succeeds with no .po files present
• Add a sample .po file and confirm the translated version builds to book/<lang>/
• Confirm GitHub Pages deployment succeeds

🤖 Generated with Claude Code