Add example of multiple nodes on same VM

[Dan-DeAraujo]

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Description

It's not clear from the documentation that the container will be listening on a port other than 5555 when the SE_NODE_PORT parameter is provided. This PR adds an example of the case where there are two nodes running on the same machine/VM, and shows the correct port mapping and environment variable configuration

Motivation and Context

This behavior is different than in Selenium3, where the container was always listening on 5555, so the port mapping always pointed there. TBH, I'm not 100% sure if this was an intended change or not... but if not, please let me know and I will close update this PR and open a bug report.
Note that this is also different behavior than the VNC port for the container, which always listens on 5900.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.

[diemol]

Thank you, @Dan-DeAraujo!