[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/support/src/migrations.rs

@@ -441,6 +441,16 @@ pub enum SteppedMigrationError {
 	Failed,
 }
 
+/// A generic migration identifier that can be used by MBMs.
+///
+/// It is not required that migrations use this identifier type, but it can help.
+#[derive(MaxEncodedLen, Encode, Decode)]
+pub struct MigrationId<const N: usize> {
+	pub pallet_id: [u8; N],
+	pub version_from: u8,
+	pub version_to: u8,
+}
+
 /// Notification handler for status updates regarding Multi-Block-Migrations.
 #[impl_trait_for_tuples::impl_for_tuples(8)]
 pub trait MigrationStatusHandler {