There is no documentation specific to adding Universal Email to an Upload page.
the Help Documentation for Universal Email is available here:
universal-email/
First, you will need to modify the Insert Record server behavior to make sure it does not redirect. Go to window -> Server Behaviors to open the server behaviors panel, then double click the Insert Record behavior and unset the Redirect after insert option.
to add Universal Email, go to Insert -> WebAssist -> Universal Email.