docs: move security article to a new 'articles' section

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: move 'vagrant' install docs to the more accurate 'mac' install docs

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: change topic of windows install docs to 'install docker on windows'

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: instead of a top-level 'upgrading docker' doc, add an 'upgrades' paragraph to each installation target

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: remove out-of-date kernel requirements document.

Requirements differ from platform to platform depending on the plugins used, and so should be documented separately for each target platform

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: reuse official dependencies list from packager's guide for custom binary install docs
Merge pull request #3618 from shykes/docs-cleanup-install

Docs cleanup install