chore: Update devnet docs

[netrome]

This PR contains three small updates to improve the experience using our devnet CLI:

1. I've added installation instructions for Terraform and Gcloud, which are needed to run the devnet CLI.
2. I included the devnet crate in the root workspace and renames the binary to mpc-devnet.
3. I added installation instructions for the CLI and changed the examples to use the mpc-devnet command directly instead of the verbose target/debug/devnet command.

[ghost]

Thanks for adding it to the workspace. Can you delete the old lock file as it is no longer used? https://github.com/near/mpc/blob/chore/devnet_experiment/devnet/Cargo.lock

[netrome]

Thanks for adding it to the workspace. Can you delete the old lock file as it is no longer used? https://github.com/near/mpc/blob/chore/devnet_experiment/devnet/Cargo.lock

Good point, will do