fixed broken links in docs

I need this to be merge to solve that issue: docker/docs#1068, and merge new CI tests for the docs: docker/docs#1052

Signed-off-by: Adrien Duermael <adrien@duermael.com>

docs/reference/commandline/login.md

@@ -29,7 +29,7 @@ adding the server name.
 `docker login` requires user to use `sudo` or be `root`, except when:
 
 1. connecting to a remote daemon, such as a `docker-machine` provisioned `docker engine`.
-2. user is added to the `docker` group. This will impact the security of your system; the `docker` group is `root` equivalent. See [Docker Daemon Attack Surface](/security/security/#docker-daemon-attack-surface) for details.
+2. user is added to the `docker` group. This will impact the security of your system; the `docker` group is `root` equivalent. See [Docker Daemon Attack Surface](/engine/security/security/#docker-daemon-attack-surface) for details.
 
 You can log into any public or private repository for which you have
 credentials. When you log in, the command stores encoded credentials in

docs/understanding-docker.md

@@ -208,8 +208,8 @@ existing images and pull them from the registry to a host.
 [Docker Hub](http://hub.docker.com) is a public Docker
 registry which serves a huge collection of existing images and allows you to
 contribute your own. For more information, go to
-[Docker Registry](https://docs.docker.com/registry/overview/) and
-[Docker Trusted Registry](https://docs.docker.com/docker-trusted-registry/overview/).
+[Docker Registry](https://docs.docker.com/registry/) and
+[Docker Trusted Registry](https://docs.docker.com/datacenter/dtr/2.0/).
 
 [Docker store](http://store.docker.com) allows you to buy and sell Docker images.
 For image, you can buy a Docker image containing an application or service from