Document SSH connector usage for docker-plugin in JCasC (fixes #2739)

[hyderraza18]

This PR adds a new demo showing how to configure the docker-plugin with the SSH connector using Jenkins Configuration as Code (JCasC). The existing documentation does not include an SSH example for docker-based agents, and the structure of this connector is not obvious from plugin usage, so this demo is added to help users configure it correctly.

The new files include:

demos/docker-plugin/docker-plugin-ssh.yaml : a minimal working SSH connector example

demos/docker-plugin/README.md

This example uses the proper sshConnector field and a Jenkins credential ID for the SSH private key, which resolves the issue reported in #2739.

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[hyderraza18]

@timja Thanks the feedback; will try to it