cpufreq: intel_pstate: Document the current behavior and user interface

Add a document describing the current behavior and user space
interface of the intel_pstate driver in the RST format and
drop the existing outdated intel_pstate.txt document.

Also update admin-guide/pm/cpufreq.rst with proper RST references
to the new intel_pstate.rst document.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>