docs: simplify basic usage doc, step 1: check your docker install

docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
docs: move a sysadmin-focused topic out of the basic usage manual and into installation docs (setting up a user group for non-root access)

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)