Document all rpc requests made by the extension

We currently have no comprehensive documentation on all the request the extension makes to the network provider.

This would be helpful to infura to understand how they can help improve metamask. It would also be helpful to people building layer 2s and other chains, to understand the functionality their chain needs to support to integrate with MetaMask. It would also help anyone developing a network provider for existing change.

The output of this task should be a file added to the `docs` directory of this repo. It should identify every request, where in the code the final `fetch` call is made, and where in the call stack the request originates. It should also describe the need/purpose of the request.

Start by surveying the extension's network requests and tracing each one to the final fetch call and its originating call stack. Add a file under docs that catalogs every request and explains its purpose and provider requirements; done means all requests are covered. No specific files or tests are named, so the initial work is repository-wide investigation.

Written by the indexing model from the issue text.