Add a document similar to naming SupportTrainingService.md

[rabbit008]

Short summary about the issue/question:
Can we add a summary summary page about trainingservices, similar to BuiltinTuner.md
https://github.com/microsoft/nni/blob/master/docs/en_US/Tuner/BuiltinTuner.md
Brief what process you are following:

How to reproduce it:
Separate the contents of the HowToImplementTrainingService.md file, and create a new file called SupportTrainingService.md, which introduces the definition of trainingservice and the the TrainingService we have now, then puts it in the same directory.

nni Environment:

• nni version:
• nni mode(local|pai|remote):
• OS:
• python version:
• is conda or virtualenv used?:
• is running in docker?:

need to update document(yes/no):

Anything else we need to know: