docs: add docker-plugin SSH connector JCasC example (fixes #2739)

Author: hyderraza18

demos/docker-plugin/README.md

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+# Docker Plugin — SSH Connector JCasC Demo
+
+This demo provides a working Jenkins Configuration as Code (JCasC) example for configuring
+the Docker plugin with the SSH connector. The existing documentation does not show how to
+set up SSH-based agents using the docker-plugin, so this example is added to fill that gap.
+
+## SSH connector usage
+
+The docker-plugin exposes the `sshConnector` field for configuring SSH-based cloud agents.
+Using other keys such as `ssh:` does not match the expected structure and will cause a
+configuration error.
+
+A working configuration can be found in `docker-plugin-ssh.yaml`. The key part of the
+connector configuration is:
+
+```yaml
+connector:
+  sshConnector:
+    sshKeyCredentialsId: "my-ssh-key-id"
+    javaPath: "/opt/java/openjdk/bin/java"

demos/docker-plugin/docker-plugin.ssh.yaml

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+# demos/docker-plugin/docker-plugin-ssh.yaml
+jenkins:
+  clouds:
+    - docker:
+        name: "my-docker-cloud"
+        dockerHost:
+          uri: "unix:///var/run/docker.sock"
+        templates:
+          - labelString: "worker1-agent"
+            dockerTemplateBase:
+              image: "jenkins/ssh-agent:latest-jdk17"
+              mounts:
+                - "type=tmpfs,destination=/run"
+                - "type=bind,source=/var/run/docker.sock,destination=/var/run/docker.sock"
+
+            remoteFs: "/home/jenkins/agent"
+            connector:
+              sshConnector:
+                sshKeyCredentialsId: "my-ssh-key-id"
+                javaPath: "/opt/java/openjdk/bin/java"