Runtime Upgrade ref docs and Single Block Migration example pallet (#1554)

Closes https://github.com/paritytech/polkadot-sdk-docs/issues/55

- Changes 'current storage version' terminology to less ambiguous
'in-code storage version' (suggestion by @ggwpez)
- Adds a new example pallet `pallet-example-single-block-migrations`
- Adds a new reference doc to replace
https://docs.substrate.io/maintain/runtime-upgrades/ (temporarily living
in the pallet while we wait for developer hub PR to merge)
- Adds documentation for the `storage_alias` macro
- Improves `trait Hooks` docs 
- Improves `trait GetStorageVersion` docs
- Update the suggested patterns for using `VersionedMigration`, so that
version unchecked migrations are never exported
- Prevents accidental usage of version unchecked migrations in runtimes

https://github.com/paritytech/substrate/pull/14421#discussion_r1255467895
- Unversioned migration code is kept inside `mod version_unchecked`,
versioned code is kept in `pub mod versioned`
- It is necessary to use modules to limit visibility because the inner
migration must be `pub`. See
https://github.com/rust-lang/rust/issues/30905 and

https://internals.rust-lang.org/t/lang-team-minutes-private-in-public-rules/4504/40
for more.

### todo

- [x] move to reference docs to proper place within sdk-docs (now that
https://github.com/paritytech/polkadot-sdk/pull/2102 is merged)
- [x] prdoc

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Juan <juangirini@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: command-bot <>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>

substrate/frame/nfts/src/lib.rs

@@ -76,7 +76,7 @@ pub mod pallet {
 	use frame_support::{pallet_prelude::*, traits::ExistenceRequirement};
 	use frame_system::pallet_prelude::*;
 
-	/// The current storage version.
+	/// The in-code storage version.
 	const STORAGE_VERSION: StorageVersion = StorageVersion::new(1);
 
 	#[pallet::pallet]

substrate/frame/nfts/src/migration.rs

@@ -54,17 +54,17 @@ pub mod v1 {
 	pub struct MigrateToV1<T>(core::marker::PhantomData<T>);
 	impl<T: Config> OnRuntimeUpgrade for MigrateToV1<T> {
 		fn on_runtime_upgrade() -> Weight {
-			let current_version = Pallet::<T>::current_storage_version();
-			let onchain_version = Pallet::<T>::on_chain_storage_version();
+			let in_code_version = Pallet::<T>::in_code_storage_version();
+			let on_chain_version = Pallet::<T>::on_chain_storage_version();
 
 			log::info!(
 				target: LOG_TARGET,
-				"Running migration with current storage version {:?} / onchain {:?}",
-				current_version,
-				onchain_version
+				"Running migration with in-code storage version {:?} / onchain {:?}",
+				in_code_version,
+				on_chain_version
 			);
 
-			if onchain_version == 0 && current_version == 1 {
+			if on_chain_version == 0 && in_code_version == 1 {
 				let mut translated = 0u64;
 				let mut configs_iterated = 0u64;
 				Collection::<T>::translate::<
@@ -77,13 +77,13 @@ pub mod v1 {
 					Some(old_value.migrate_to_v1(item_configs))
 				});
 
-				current_version.put::<Pallet<T>>();
+				in_code_version.put::<Pallet<T>>();
 
 				log::info!(
 					target: LOG_TARGET,
 					"Upgraded {} records, storage to version {:?}",
 					translated,
-					current_version
+					in_code_version
 				);
 				T::DbWeight::get().reads_writes(translated + configs_iterated + 1, translated + 1)
 			} else {