docs: add code examples to migrating_api article

[noliveleger]

Summary

• Added an Authentication section explaining the Authorization: Token xxxx header with curl, Python (requests), and R (httr) examples
• Added code examples (curl / Python / R) to every endpoint section: project list, submission data, form details, tags, permissions, metadata, and user profile
• Restructured each section with #### Code examples before #### Response examples for faster scanning
• Wrapped response examples in <details> blocks to reduce visual noise
• Fixed a Markdown rendering bug in the Form endpoints field mapping table (missing blank line before table)
• Updated Last updated date to 19 May 2026 (commit ID still need to be changed)
• Applied all changes to the French, Spanish, and Arabic locales