NVML support can't be disabled in the build system
If the user wants to disable NVML support (even though all dependencies are found by CMake), this is not possible without editing the cache manually.
Allowed NVML support to be toggled
A new CMake advanced option GMX_USE_NVML takes its default value from
the result of the NVML detection, thus by default NVML support is on
only if the detection succeeds. It can be turned off or on at will,
but when set on, it will give a fatal error if the detection did not
Split the NVML section of the install guide into its own paragraph.