doc: move device binding up in Linux guide

doc: move device binding up in Linux guide

While the details of VFIO and UIO may be of interest to some, most users
of the doc are likely primarily interested in how to bind their devices
to the kernel driver and then move on to running the app. Therefore, the
most important part of the "Linux Drivers" section of the GSG is the
subsection on "Binding and Unbinding", so put that first.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>