Docs: articles/certificates.md rewrite.

This commit contains a re-structured re-write of the original
certificated.md file, containing the amendments proposed with
PR #7120 (commit ID bd28595e310ccfa98) by @timthelion.

Related to: https://github.com/dotcloud/docker/pull/7120

2014-07-30: Update/rebase/squash based on the comments from @jamtur01 and @fredlf.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)