docs: add PCIe devices placement guidelines

docs: add PCIe devices placement guidelines

Proposes best practices on how to use PCI Express/PCI device
in PCI Express based machines and explain the reasoning behind them.

Reviewed-by: Laszlo Ersek <[email protected]>
Signed-off-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>