DX: improve Mina Signer docs

[barriebyron]

zkIgnite Cohort 1 Technical Feedback identified an issue that has been only lightly fixed and requires more content.
O(1) Copy of [Open] Cohort 1 Technical Feedback
SEE ROW 2
The mina-signer package for key pair generation and transaction signing
related to (duplicate of?) https://app.zenhub.com/workspaces/zkapps-product-eng-6130fedb3b0fc600123d8796/issues/gh/o1-labs/docs2/257

Let's answer these questions:

• What is Mina Signer and how is it used?
• What Mina Signer content do devs need to succeed?
• Can we provide example code?

https://docs.minaprotocol.com/node-operators/mina-signer supersedes the deprecated Client SDK doc but is incomplete and can be more helpful

See o1-labs/o1js#1026 for devs to use mina-signer to create Nullifiers in production, since it does not leak the private key of the user.

#443 introduced a quick fix:

• Replaced the current Client SDK topic with a new Mina Signer topic that links to the repo with install and usage instructions

• Updated the Node Operator FAQ topic to describe Mina Signer and mention deprecation, add description of zkApp, and update known errors

• MF view only OPEN Cohort 1 Technical Feedback

• O(1) Copy of OPEN Cohort 1 Technical Feedback sheet

[barriebyron]

potential for an explainer doc in the zkApp dev docs section
how to use Mina Signer with zkApp