docs: cdrom-standard.tex: convert from LaTeX to ReST

This is the only LaTeX documentation file inside the documentation.

Instead of having a Latex document directly there, convert
it to ReST format, as this is the format we're using for docs.

For now, let's keep the extension as .txt in order to avoid
warnings when building the documentation with Sphinx.

The next patch patch will rename it to .rst and add it to the
building system.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>