Add documentation for .getType() vs .getTypeNode()

As title. Seems like this is a common point of confusion.

Might be nice to have a look through the documentation to give people a bit more of an intro to the TypeScript Compiler API and what ts-morph does under the hood.

Start by reviewing the existing documentation for .getType() and .getTypeNode(), then read the relevant TypeScript Compiler API material to understand the distinction and ts-morph’s underlying model. Done means the documentation explains when to use each method and gives newcomers a useful introduction to the related compiler API concepts.

Written by the indexing model from the issue text.