Documentation: admin-guide: Move intel_powerclamp documentation

Create a folder "thermal" under Documentation/admin-guide and move
intel_powerclamp documentation to this folder.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>