Update Intro for Infura docs to reflect MM context

[joaniefromtheblock]

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 though