Add external translation provider workflow guide

[mintlify]

Summary

Adds documentation for teams using their own translation providers (external vendors, regional translators) to work with Mintlify documentation.

• Workflow overview for export/import translation cycles
• GitHub Actions examples for automating content export and translation import
• Best practices for working with external translators (frontmatter, code blocks, glossaries)

Addresses the need outlined in DOC-263 for customers like Notion who work with different regional translators and need guidance on getting MDX files to/from their translation providers.

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Note

Low Risk
Documentation-only change; adds examples and guidance without modifying product code or runtime behavior.

Overview
Adds a new External translation providers section to guides/internationalization.mdx, describing how to run a vendor/region-based translation workflow alongside Mintlify docs.

Includes a high-level export/import cycle, sample GitHub Actions for exporting changed source .mdx files on main and validating translated PRs (frontmatter + structure checks), plus best practices for protecting frontmatter/code blocks and maintaining translation consistency.

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