This adds a NixOS option for setting the CPU max and min frequencies
with `cpufreq`. The two options that have been added are:
- `powerManagement.cpufreq.max`
- `powerManagement.cpufreq.min`
It also adds an alias to the `powerManagement.cpuFreqGovernor` option as
`powerManagement.cpufreq.governor`. This updates the installer to use
the new option name. It also updates the manual with a note about
the new name.
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