docs: cpu-freq: convert index.txt to ReST

most of the stuff there can be re-used with ReST format,
but we need to add an empty TOC and remove the existing
entries, as the following conversion patches will be re-adding
them, as they're converted.

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