Implement i18n

[stkossman]

Implement i18n (Ukrainian & English).

Set up Starlight's built-in internationalization to support English as the default language and Ukrainian as an additional option.

Tasks:

• Update astro.config.mjs to configure the locales object (root for English, uk for Ukrainian).
• Migrate existing content in src/content/docs/ to src/content/docs/en/.
• Create the src/content/docs/uk/ directory structure.
• Translate the landing page (index.mdx) and the sidebar navigation configuration into Ukrainian.
• Ensure the language switcher appears and functions correctly in the global header.