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: 707bf8e1dfd51d ("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>
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index c872a8a..0571938 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -116,7 +116,7 @@
    svga
    syscall-user-dispatch
    sysrq
-   thermal
+   thermal/index
    thunderbolt
    ufs
    unicode
diff --git a/Documentation/admin-guide/thermal/index.rst b/Documentation/admin-guide/thermal/index.rst
new file mode 100644
index 0000000..193b7b0
--- /dev/null
+++ b/Documentation/admin-guide/thermal/index.rst
@@ -0,0 +1,8 @@
+=================
+Thermal Subsystem
+=================
+
+.. toctree::
+   :maxdepth: 1
+
+   intel_powerclamp