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: Nkernel test robot <lkp@intel.com>
Signed-off-by: NBagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/admin-guide/index.rst

@@ -116,7 +116,7 @@ configure specific aspects of kernel behavior to your liking.
    svga
    syscall-user-dispatch
    sysrq
-   thermal
+   thermal/index
    thunderbolt
    ufs
    unicode

Documentation/admin-guide/thermal/index.rst

+=================
+Thermal Subsystem
+=================
+
+.. toctree::
+   :maxdepth: 1
+
+   intel_powerclamp