Add documentation that maps and explains all Dapp API implementation related code

[danjm]

As of today, implementation of API methods is spread across multiple repos. The code within the extension code base is spread across multiple files. We eventually want to consolidate this and organize it better, but in the meantime it would be good if we could document where all code related to the dapp api is found.

This can include code that ultimately handles each method request, as well as the code that makes up the infrastructure / request handling system.

Architecture diagrams could help as well.