Update documentation for Docker Compose V2 (i.e. compose plugin)

Docker now officially supports Docker Compose in the form of a plugin
for the docker command.
See: https://www.docker.com/blog/announcing-compose-v2-general-availability/

Following the official Docker documentation for Linux will install the
Docker Compose plugin. We can remove the mention of the docker-compose
Debian package.

Command lines and pointers to the documentation are now updated
accordingly.