Docs: articles/certificates.md rewrite.

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)
Simplify: The presence of one or more `<filename>.key/cert` pairs indicates to Docker that there are custom certificates required for access to the desired repository.

The editorial principle here is to simplify clauses to a single, clear subject-verb-object (actor-action-acted upon) statement.