Better documentation of open hardware licenses and file availablity

The [FAQ](https://www.arduino.cc/en/Main/FAQ#toc3) indicates the design files are licensed under a CC-BY-SA license. However, when I go to [a product page](https://store.arduino.cc/usa/arduino-due) I can download the schematic files but there is no indication of license or license terms there, nor have I been able to find anywhere saying which ***specific*** license the files are under and I am left not knowing which license I would need to adhere to.

Start by reviewing the linked FAQ and Arduino Due product page, then identify where the specific open-hardware license and file availability should be documented. Done means the relevant documentation clearly names the license terms and tells users where the design files are available.

Written by the indexing model from the issue text.