docs: Clarify Asset vs File endpoints and document supported sources for URL-based uploads

[DialloWill]

Closes #754

This PR adds a new documentation page addressing the ambiguity reported in issue #754 regarding the /api/assets endpoint versus the legacy file upload endpoints.

What's added:

• Clear distinction between the Assets API and File API with a comparison table
• Explicit definition of "supported sources" for URL-based uploads
• Common failure scenarios for UNSUPPORTED_SOURCE errors with solutions
• Code examples for both upload methods (direct file and URL-based)
• Step-by-step guide for using an uploaded asset as a workflow input
• Explanation of deduplication behavior (content-addressed storage)
• Decision tree helping users choose the right upload method

Why:
Users were receiving UNSUPPORTED_SOURCE errors with no guidance on what sources are allowed. It was also unclear whether assets uploaded via /api/assets could be used as workflow inputs or if users should use the file endpoints instead.