Documentation: admin-guide: Add toctree entry for thermal docs

kernel test robot reported htmldocs warnings:

Documentation/admin-guide/index.rst:62: WARNING: toctree contains reference to nonexisting document 'admin-guide/thermal'
Documentation/admin-guide/thermal/intel_powerclamp.rst: WARNING: document isn't included in any toctree

Add toctree entry for thermal/ docs to fix these warnings.

Link: https://lore.kernel.org/linux-doc/202302121759.MmJgDTxc-lkp@intel.com/


Fixes: 707bf8e1 ("Documentation: admin-guide: Move intel_powerclamp documentation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>