Docs: articles/certificates.md rewrite - initial pass for review

Docs: articles/certificates.md rewrite - initial pass for review.

This commit contains a re-structured re-write of the original
certificated.md file, [also] containing the amendments proposed with
PR #7120 (commit ID bd28595e310ccfa98) by @timthelion.

Related to: https://github.com/dotcloud/docker/pull/7120

Please consider this as an initial pass for *your* review.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
On the client? You've lost me. Are these client certs local to my Docker client? Or on the Docker registry host? Or...?