Added test manager documentation.

[Jeffrey-Simpson]

Added test manager information to the read the docs documentation.

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This change is 

[craig8]

Please add a preamble to tell us exactly what the test manager allows us to do....as you can see from the screen shot of the built docs just going into the configuration seems like an abrupt transition.

Note if this is elsewhere in the docs please reference this here.

[Jeffrey-Simpson]

I have updated and resolved the comments but I still see a change requested. Is there something else I need to do?

[AAndersn]

@poorva1209 I suggest closing this old ticket

This has already been covered in new docs:

https://gridappsd-training.readthedocs.io/en/latest/api_usage/3.5-Creating-Running-Simulation-API.html#Test-Manager-Configuration

https://gridappsd-training.readthedocs.io/en/latest/installation/1.5-Using-GridAPPS-D-Viz.html#Test-Manager-Configuration