Update caching docs

[treuille]

Summary

As part of our new caching plan for December we have written new docs.

Tasks:

• Copy edit and covnert the new caching docs to markdown and insert into the site
• Make sure that all errors and warninng that refer to the docs are linked properly: Throw a new UnhashableTypeError if all other attempts to hash an obect (aka __reduce__) fail #768 Update the Cached Object Mutated warning #773 Update the Input Key Mutated warning #774
• Make all the other docs flow properly with this