Created documentation for the cache class

[andrtang]

This PR adds documentation for the Cache class to make it easier to understand for users and developers. It can act as a template for documentation for future classes and includes the following:

• A summary of the cache class
• A summary of all the attributes, methods, and internal methods in the cache class
• An example of how the cache class can be utilized

The PR also includes a docs folder to allow for other developers to add or update documentation for other classes in wig.