Replace all tb.ref examples in docs with tb.get

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 piece of code outlines the different ways we can create testbook reference objects as of now.