feat: Add device-opcua documentation

[jiekechoo]

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[lenny-goodell]

@jiekechoo , the OPCUA device service in holding is NOT a support EdgeX service so I have put this PR in draft. I assume your intention is to rework that service and propose it be accepted into EdgeX?

[jiekechoo]

@lenny-intel

the OPCUA device service in holding is NOT a support EdgeX service so I have put this PR in draft.

Ok,it's all right.

I assume your intention is to rework that service and propose it be accepted into EdgeX?

Yes. I'll contribute several device services, I believe that more users will need them.

[FelixTing]

@jiekechoo I noticed that the README mentions the limitation that this device service is currently limited to a single device instance. Could you also include this note in the documentation for consistency? It would be helpful for users.