[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>

Cargo.lock

@@ -7053,6 +7053,7 @@ dependencies = [
  "pallet-election-provider-multi-phase",
  "pallet-election-provider-support-benchmarking",
  "pallet-elections-phragmen",
+ "pallet-example-mbm",
  "pallet-example-tasks",
  "pallet-fast-unstake",
  "pallet-glutton",
@@ -9837,6 +9838,20 @@ dependencies = [
  "sp-std 14.0.0",
 ]
 
+[[package]]
+name = "pallet-example-mbm"
+version = "0.1.0"
+dependencies = [
+ "frame-benchmarking",
+ "frame-support",
+ "frame-system",
+ "log",
+ "pallet-migrations",
+ "parity-scale-codec",
+ "scale-info",
+ "sp-io",
+]
+
 [[package]]
 name = "pallet-example-offchain-worker"
 version = "28.0.0"