Update Intro for Infura docs to reflect MM context

Make updates to intro to add greater context related to Infura as RPC nodes (there is already some there). Also, add an architecture diagram to better illustrate how this all works (a simplified overview):


* Set Infura as the RPC provider in MetaMask.
* Secure transactions as MetaMask handles signing, Infura handles broadcasting.
* Infura provides scalable access to blockchain data.
* Easily switch between networks using Infura endpoints.

I think this is [still in development](https://docs.google.com/document/d/1ZSIhaUQzWkcURogxIHSxC3N641dghw0RWHChq5-XUFA/edit#heading=h.mr8asmulvdr) though

Start by locating the existing Infura introduction in the MetaMask documentation and review the linked development document and attached architecture example. Update the introduction with the listed MetaMask, RPC, signing, broadcasting, scalability, and network-switching context, then add a simplified architecture diagram showing these relationships.

Written by the indexing model from the issue text.