The proper way to achieve this is to use the built-in AMP Output Format provided by Hugo which by default create the AMP file next to the page’s HTML file.
Now as I understand it, you need your AMP file path to be relative to your content/ directory. So your question is, how can I make this AMP file live outside the path of my page.
According to the doc on the matter, the option path defines the sub-path where the output file is gonna live. I assume sub path of the page itself.
So I am not sure there is a way at the moment to achieve what you want. I would suggest you rename this thread to (or create a new one named) : Defining Ouput Format path relative to content/ rather than page’s sub-path