remote: document changes & new commands

[preminger]

Once these issues, relating to reorganizing the functionality under the remote, have been closed —

• move keyserver-related operations from remote to new keyserver command singularity#1893
• move OCI registry operations from remote to new registry command singularity#1894
• automatically select argument to remote add as current remote singularity#1895
• "content-centric" format of remote list output, instead of table singularity#1896

— the documentation in https://docs.sylabs.io/guides/latest/user-guide/endpoint.html should be modified & reorganized accordingly.

Discussion: sylabs/singularity#1639