doc: thermal: add cpu-idle-cooling to index tree

Add cpu-idle-cooling.rst to the index list for user reference through
self-compiled htmldocs or online documentation tree of kernel.

Like: https://www.kernel.org/doc/html/latest/driver-api/thermal/index.htmlSigned-off-by: Wang Wenhu <wenhu.wang@vivo.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200429131109.80891-1-wenhu.wang@vivo.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/thermal/cpu-idle-cooling.rst

+================
+CPU Idle Cooling
+================
 
 Situation:
 ----------

Documentation/driver-api/thermal/index.rst

@@ -8,6 +8,7 @@ Thermal
    :maxdepth: 1
 
    cpu-cooling-api
+   cpu-idle-cooling
    sysfs-api
    power_allocator