docs: trace: ring-buffer-design.txt: convert to ReST format

- Just like some media documents, this file is dual licensed
  with GPL and GFDL. As right now the GFDL SPDX definition is
  bogus (as it doesn't tell anything about invariant parts),
  let's not use SPDX here. Let's use, instead, the same test
  as we have on media.
- Convert title to ReST format;
- use :field:  markup;
- Proper mark literal blocks as such;
- Add it to trace/index.rst file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d350be9b666ca0de441b684b2282ddd76bd7b397.1592918949.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>