docs: sh: convert new-machine.txt to ReST

- Add a SPDX header;
- Adjust document title to follow ReST style;
- Mark literal blocks as such;
- Mark a table as such;
- Add it to sh/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4437d379ccf201cc3a369232f9159a02754ca530.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>