[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/mod.rs

@@ -0,0 +1,29 @@
+// 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.
+
+/// # Multi-Block Migrations Module
+///
+/// This module showcases a simple use of the multi-block migrations framework.
+pub mod v1;
+
+/// A unique identifier across all pallets.
+///
+/// This constant represents a unique identifier for the migrations of this pallet.
+/// It helps differentiate migrations for this pallet from those of others. Note that we don't
+/// directly pull the crate name from the environment, since that would change if the crate were
+/// ever to be renamed and could cause historic migrations to run again.
+pub const PALLET_MIGRATIONS_ID: &[u8; 18] = b"pallet-example-mbm";