replace mentions of `remote` cmd with `registry` where appropriate, from sylabs 215

[JasonYangShadow]

This fixes a subpart of https://github.com/apptainer/apptainer-userdocs/issues/239 by merging

• https://github.com/sylabs/singularity-userdocs/pull/215 which fixed
• https://github.com/sylabs/singularity/pull/1908

The original PR description was:

The "Support for Docker and OCI Containers" (singularity_and_docker.rst) was still referring to the remote command for functionality that has now been moved to the registry command (see https://github.com/sylabs/singularity/pull/1908).

This PR fixes those occurrences to registry, as appropriate.