If you look under the documentation’s Guides section there is a part which explains the process. I do think it needs updating, however, especially as TestFlight is now part of the Apple process and (IMHO) a bit more complex and demanding that it used to be. For example, you are required to provide all the images etc that a potentially releasable binary would need.
My process for uploading to Apple’s iTunes Connect is to build for iOS using Corona, put the .app inside a folder called Payload and then zip the folder. I then rename the resulting zip file to be whatever the .app file was called, with the .app extension renamed to .ipa - this is the process I’ve read works in the past and used to work for deploying with Apple.
Unfortunately, I can no longer upload the .ipa file using Application Loader, since TestFlight was integrated into iTunes Connect.
I hope someone can put me right and/or prove any of the above wrong.