Remove "Docker" from registry, as the registry specification is no
longer docker-specific, but part of the OCI distribution spec.
Also removed "Register" from one of the docs pages, as the login
command hasn't supported creating a new acccount on Docker Hub for
a long time.
I'm wondering if we should be more explicit about what log in / out
does (effectively; authenticate, and on success store the credentials
or token, and on log out; remove credentials/token).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>