Add template i18n documentation

[Swahjak]

Part of Notifuse/notifuse#268

Summary

• New dedicated Template Translations page covering the t filter, locale resolution, workspace translations, import/export, and best practices
• Updated templates, workspaces, contacts, transactional API, and broadcast campaigns pages with i18n cross-references
• Added workspace translations API endpoints and template schema changes to OpenAPI spec
• Updated Mintlify navigation with new page and API group

Related PRs

• Add template i18n with Liquid t filter notifuse#271 — Add template i18n with Liquid t filter (main implementation)
• Evaluate and pass keyword args to Liquid filter invocations liquidgo#3 — Evaluate and pass keyword args to Liquid filter invocations

Test plan

• Verify Mintlify renders features/template-translations.mdx correctly
• Verify navigation shows Template Translations after Templates in sidebar
• Verify Workspace Translations API group appears in API Reference tab
• Verify all cross-page links resolve correctly