Quick answer - don't bother, use conventional SATA, it's no slower, and you should comfortably be able to have enough SATA ports available. If the Maximus Impact's 4 SATA ports is a limitation, consider that the Asus Z87I-Deluxe has 6, and is fairly similar in specs but lacks the sound card and a few other fancy features.
Long answer -
There are 3 different mini SSD form factors, mini PCIe, mSATA and M.2. The first two are often interchangeable, as they use the same interface, but you can't take this totally for granted. The AsRock Z87E-ITX offers an interchangeable slot, but the Asus does not.
The Maximus Impact has two slots on a daughterboard (
this link, scroll to "mPCIe Combo II
with 802.11ac/Bluetooth 4.0") one is for mini PCIe and is pre-occupied by the Wifi/Bluetooth, and the M.2 slot is free.
You are right that the orientation isn't clear, but given that one end will sit flush with the I/O backplate, it's a pretty safe assumption that it points into the case, with it sitting parallel to your graphics card. The one you linked should fit, but unless you are desperate to use that motherboard and save SATA ports, I wouldn't bother.