add a push-style example to borg-create docs
When I browsed the readthedocs and manpages while setting up Borg for
myself for the first time, I saw multiple hints that pushing backups via
SSH is supported (e.g. the existence of the `borg serve` command and the
`--rsh` option), but I never found a clear example. I only found
<https://borgbackup.readthedocs.io/en/stable/usage/general.html#repository-urls>
after being pointed to it by @ThomasWaldmann.
Therefore, an explicit example in the `borg create` documentation seems
appropriate. I put it next to the pull-style example using sshfs which
serves a similar usecase.
