[doc porting] several docs
Ported to mdx:
```
docs/source/debugging.rst
docs/source/testing.rst
docs/source/main_classes/deepspeed.rst
```

fixed a few small code block breakages in the first one.

@sgugger 
Amazing that you noticed all those issues, @sgugger - I missed all of those.

It looks like we need to be careful with suggestions that include "```", it looks like github is buggy there, ate the "````" you added since it's also part of suggestion - had to switch to 4 backticks suggestion.