Add docs for leo devnet

[zklimaleo]

moving from AleoNet/welcome#77

Running a Devnet

Leo v3.1.0 provides a simple way to run a local devnet. You may run

leo devnet --help

to see the available options.

To quickly get started, you can run the following commands:

> mkdir tmp
> leo devnet --storage tmp --snarkos ./tmp/snarkos-test --features test_network --install

Devnet private keys include:

• APrivateKey1zkp2RWGDcde3efb89rjhME1VYA8QMxcxep5DShNBR6n8Yjh
• APrivateKey1zkp2GUmKbVsuc1NSj28pa1WTQuZaK5f1DQJAT6vPcHyWokG
• APrivateKey1zkpBjpEgLo4arVUkQmcLdKQMiAKGaHAQVVwmF8HQby8vdYs
• APrivateKey1zkp8CZNn3yeCseEtxuVPbDCwSyhGW6yZKUYKfgXmcpoGPWH

[WaltzOfWhispers]

Please prioritize documenting this new command if it's in the new release.

[AleoAlexander]

Closed with #510