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 <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/admin-guide/index.rst

···
    svga
    syscall-user-dispatch
    sysrq
+   thermal
    thunderbolt
    ufs
    unicode

Documentation/driver-api/thermal/index.rst

···
 
    exynos_thermal
    exynos_thermal_emulation
-   intel_powerclamp
    nouveau_thermal
    x86_pkg_temperature_thermal
    intel_dptf

MAINTAINERS

···
 Q:	https://patchwork.kernel.org/project/linux-pm/list/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git thermal
 F:	Documentation/ABI/testing/sysfs-class-thermal
+F:	Documentation/admin-guide/thermal/
 F:	Documentation/devicetree/bindings/thermal/
 F:	Documentation/driver-api/thermal/
 F:	drivers/thermal/