[doc] Move system requirements out of Xilinx section
[doc] Move system requirements out of Xilinx section

The system requirements got somehow moved into the Xilinx section. Move
them out to the toplevel again, as they apply to all our setup
instructions.

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan