[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>

prdoc/pr_2119.prdoc

@@ -0,0 +1,16 @@
+title: "Add example pallet for Multi-Block-Migrations"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      - Add an example pallet to demonstrate Multi-Block-Migrations.
+      - Add a `MigrationId` to frame-support for more convenient identification or migrations.
+      - Add default config prelude for testing in pallet-migrations.
+
+crates:
+  - name: frame-support
+    bump: minor
+  - name: pallet-migrations
+    bump: minor
+  - name: kitchensink-runtime
+    bump: patch