Adapt tramp-docker integration

Adapt tramp-docker integration

* doc/misc/tramp.texi (Inline methods): Add docker.
(Customizing Methods): Remove docker-tramp.  Adapt references to this.

* etc/NEWS: Mention new Tramp method "docker".  Fix typos.

* lisp/net/tramp-compat.el (docker-tramp): Warn, if that package is used.

* lisp/net/tramp-docker.el: Format header lines.
(tramp-docker): Remove custom group.
(tramp-docker-program, tramp-docker-method): Add
;;;###tramp-autoload cookie.
(tramp-docker-program): Change group to `tramp'.  Add version.
(tramp-docker--completion-function): Fix docstring.
(tramp-docker--add-method, tramp-docker--remove-method)
(tramp-docker-unload-function, tramp-docker-setup): Remove functions.
(tramp-methods, tramp-set-completion-function): Initialize via
`tramp--with-startup'.
(tramp-unload-hook):  Unload `tramp-docker'.