text: Title: Replace all tb.ref examples in docs with tb.get
  Body: With #92, a new way of creating reference objects was introduced. 
  
  - We will need to replace all instances of `tb.ref` with `tb.get` and also mention that one can use the indexing pattern to create the objects.
  - Also mention that when the object is JSON serializable,  when `tb.get` is called on that variable, the actual value will be fetched, and not the reference object, which was the case before. (This is the actual reason why we need to move away from `tb.ref` examples because they no longer return "references" always.)
  - Provide a few code examples to demonstrate the new pattern.
  
  [This](https://github.com/nteract/testbook/blob/main/testbook/client.py#L34-L51) piece of code outlines the different ways we can create testbook reference objects as of now.