Document the kagami validator feature and its behavior

[6r1d]

It deserves to be documented, and I'll address that.

Moreover, the default CLI help can be better.

./target/debug/kagami validator --help
Generate the default validator

Usage: kagami validator [OPTIONS]

Options:
      --clone-dir <CLONE_DIR>  Directory path to clone Iroha sources to. Only used in case when `kagami` is run as a separate binary. A temporary directory is created for this purpose if this option is not provided but cloning is still needed
  -h, --help                   Print help

The Validator feature required to use the fresh toolchain, add rust-src and WASM target support:

rustup default nightly-2023-06-25
rustup component add rust-src --toolchain nightly-2023-06-25
rustup target add wasm32-unknown-unknown --toolchain nightly-2023-06-25

[0x009922]

Closed becase kagami validator doesn't exist anymore:

• [suggestion] Separate validator and swarm from Kagami iroha#3833
• [refactor] #3833, #2373, #3601: Split off Kagami iroha#3841