wifi example doc fixes

[knmcguire]

There are some fixes to be done to the documentation of the wifi example:

• [ ] The container name in the getting started tutorial is not the same as the one used here.
• [ ] For the opencv viewer, the regular opencv pip package needs to be used, but currently this interferes with the opencv headless of the cfclient. It needs to be explained that this needs to be installed but in a virtual environment.
• [ ] There is no explanation yet about when you would need to input the IP address in the opencv-viewer.py or not. This should probably be a separate guide that needs to be referred too.
• [ ] A picture of opencv viewer?

[knmcguire]

Also see this current issue for the tutorial: https://github.com/bitcraze/bitcraze-website/issues/127