[doc] Example MBM pallet (#2119)

## Basic example showcasing a migration using the MBM framework

This PR has been built on top of
https://github.com/paritytech/polkadot-sdk/pull/1781 and adds two new
example crates to the `examples` pallet

### Changes Made:

Added the `pallet-example-mbm` crate: This crate provides a minimal
example of a pallet that uses MBM. It showcases a storage migration
where values are migrated from a `u32` to a `u64`.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/examples/multi-block-migrations/src/migrations/v1/benchmarks.rs

@@ -0,0 +1,54 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Benchmark the multi-block-migration.
+
+#![cfg(feature = "runtime-benchmarks")]
+
+use crate::{
+	migrations::{
+		v1,
+		v1::{weights, weights::WeightInfo},
+	},
+	Config, Pallet,
+};
+use frame_benchmarking::v2::*;
+use frame_support::{migrations::SteppedMigration, weights::WeightMeter};
+
+#[benchmarks]
+mod benches {
+	use super::*;
+
+	/// Benchmark a single step of the `v1::LazyMigrationV1` migration.
+	#[benchmark]
+	fn step() {
+		v1::v0::MyMap::<T>::insert(0, 0);
+		let mut meter = WeightMeter::new();
+
+		#[block]
+		{
+			v1::LazyMigrationV1::<T, weights::SubstrateWeight<T>>::step(None, &mut meter).unwrap();
+		}
+
+		// Check that the new storage is decodable:
+		assert_eq!(crate::MyMap::<T>::get(0), Some(0));
+		// uses twice the weight once for migration and then for checking if there is another key.
+		assert_eq!(meter.consumed(), weights::SubstrateWeight::<T>::step() * 2);
+	}
+
+	impl_benchmark_test_suite!(Pallet, crate::mock::new_test_ext(), crate::mock::Runtime);
+}