[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/bin/node/runtime/src/lib.rs

@@ -320,6 +320,8 @@ impl pallet_example_tasks::Config for Runtime {
 	type WeightInfo = pallet_example_tasks::weights::SubstrateWeight<Runtime>;
 }
 
+impl pallet_example_mbm::Config for Runtime {}
+
 impl pallet_utility::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type RuntimeCall = RuntimeCall;
@@ -2439,6 +2441,9 @@ mod runtime {
 
 	#[runtime::pallet_index(77)]
 	pub type SkipFeelessPayment = pallet_skip_feeless_payment;
+
+	#[runtime::pallet_index(78)]
+	pub type PalletExampleMbms = pallet_example_mbm;
 }
 
 /// The address format for describing accounts.
@@ -2597,6 +2602,7 @@ mod benches {
 		[pallet_whitelist, Whitelist]
 		[pallet_tx_pause, TxPause]
 		[pallet_safe_mode, SafeMode]
+		[pallet_example_mbm, PalletExampleMbms]
 	);
 }