Rewrite the file import guide

The import documentation now explains every supported format, how to import from the dashboard or API, and how to convert an unsupported format yourself.

• Added a clear breakdown of structured and tabular formats with a brand icon for each.
• Added a section on writing a converter with the REST API, a typed SDK, or a coding agent when your format isn't supported natively.
• Updated the API reference with the new structured and tabular import endpoints.