Separate dependencies for DGX OS devices

[raziel2001au]

A recent addition to requirements.txt conflicted with the list of dependencies for DGX OS, causing the installation to fail on devices like the DGX Spark. After speaking to Ostris, we decided the best approach is to remove the dependency on requirements.txt, and instead maintain the list of dependencies separately for DGX OS devices. I also did some minor tweaks to the instructions, removing the $, which looks nice, but makes copying and pasting commands harder.