Update architecture diagram

Diagram for "Run Web3Signer at scale" needs to be updated as it is not entirely accurate. Update [the diagram](https://github.com/Consensys/doc.web3signer/blob/main/static/img/transparent_background_diagram.png), rename it for accessibility, and add to docs. 

Related to [this PR](https://github.com/Consensys/doc.web3signer/pull/254/commits/e01f419d01f88986bfaa6cd903c28e584096654d)