rust-sdk: start adding rustdoc (#2895)

sdk/rust/nym-sdk/examples/manually_handle_keys_and_config.rs

sdk/rust/nym-sdk/examples/persist_keys.rs

sdk/rust/nym-sdk/examples/simple.rs

sdk/rust/nym-sdk/examples/simple_but_manually_connect.rs

sdk/rust/nym-sdk/src/error.rs

sdk/rust/nym-sdk/src/lib.rs

sdk/rust/nym-sdk/src/mixnet.rs

sdk/rust/nym-sdk/src/mixnet/client.rs

sdk/rust/nym-sdk/src/mixnet/config.rs

sdk/rust/nym-sdk/src/mixnet/connection_state.rs

sdk/rust/nym-sdk/src/mixnet/keys.rs

sdk/rust/nym-sdk/src/mixnet/paths.rs