docs: cpu-freq: convert cpufreq-stats.txt to ReST

- Add a SPDX header;
- Add a document title, based on the original contents of
  cpu-freq/index.txt;
- Use lists where needed;
- Comment out the existing text-only index;
- Adjust some title marks;
- Use bold on some places;
- Mark literal blocks as such;
- Some whitespace fixes and new line breaks;
- Add it to cpu-freq/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/cpu-freq/index.rst

@@ -16,6 +16,7 @@ Author: Dominik Brodowski  <linux@brodo.de>
 
    core
    cpu-drivers
+   cpufreq-stats
 
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