[media] doc-rst: Convert ci.txt to a rst file

The old ci.txt file had a very peculiar format, with doesn't
match any markup language I know. Change it to be on ReST
format, for it to be parsed by Sphinx.

Also, as this is an old document, add a note about it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>