App Distribution: clarify upload file parameter and remove Swagger UI reference

[thiagosaucelabs]

Summary

• Clarify on POST /api/v3/builds/upload that file must be sent as a multipart file part, not a text field, and show the 400 {"error": "file is required."} returned otherwise.
• Add a curl example for the upload and a note that the API does not fetch builds from a URL, so the file must be downloaded first and sent as the file part.
• Remove the "Interactive Documentation" section that pointed to a Swagger UI without a link.

Test plan

• Confirmed no other page under docs/app-distribution references Swagger
• Previewed the API Reference page and checked the Upload Parameters table and curl example render correctly

🤖 Generated with Claude Code

[github-actions]

Deploy preview ready for 3717!
https://docs.dev.saucelabs.net/pr-preview/pr-3717

[github-actions]

Deploy preview ready for 3717!
https://docs.dev.saucelabs.net/pr-preview/pr-3717