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/contracts/src/migration/v15.rs

@@ -46,7 +46,7 @@ use sp_runtime::{traits::Zero, Saturating};
 #[cfg(feature = "try-runtime")]
 use sp_std::vec::Vec;
 
-mod old {
+mod v14 {
 	use super::*;
 
 	#[derive(
@@ -81,7 +81,7 @@ pub fn store_old_contract_info<T: Config>(account: T::AccountId, info: crate::Co
 	let entropy = (b"contract_depo_v1", account.clone()).using_encoded(T::Hashing::hash);
 	let deposit_account = Decode::decode(&mut TrailingZeroInput::new(entropy.as_ref()))
 		.expect("infinite length input; no invalid inputs for type; qed");
-	let info = old::ContractInfo {
+	let info = v14::ContractInfo {
 		trie_id: info.trie_id.clone(),
 		deposit_account,
 		code_hash: info.code_hash,
@@ -92,7 +92,7 @@ pub fn store_old_contract_info<T: Config>(account: T::AccountId, info: crate::Co
 		storage_base_deposit: info.storage_base_deposit(),
 		delegate_dependencies: info.delegate_dependencies().clone(),
 	};
-	old::ContractInfoOf::<T>::insert(account, info);
+	v14::ContractInfoOf::<T>::insert(account, info);
 }
 
 #[derive(Encode, Decode, CloneNoBound, PartialEq, Eq, RuntimeDebug, TypeInfo, MaxEncodedLen)]
@@ -127,11 +127,11 @@ impl<T: Config> MigrationStep for Migration<T> {
 
 	fn step(&mut self) -> (IsFinished, Weight) {
 		let mut iter = if let Some(last_account) = self.last_account.take() {
-			old::ContractInfoOf::<T>::iter_from(old::ContractInfoOf::<T>::hashed_key_for(
+			v14::ContractInfoOf::<T>::iter_from(v14::ContractInfoOf::<T>::hashed_key_for(
 				last_account,
 			))
 		} else {
-			old::ContractInfoOf::<T>::iter()
+			v14::ContractInfoOf::<T>::iter()
 		};
 
 		if let Some((account, old_contract)) = iter.next() {
@@ -243,11 +243,11 @@ impl<T: Config> MigrationStep for Migration<T> {
 
 	#[cfg(feature = "try-runtime")]
 	fn pre_upgrade_step() -> Result<Vec<u8>, TryRuntimeError> {
-		let sample: Vec<_> = old::ContractInfoOf::<T>::iter().take(100).collect();
+		let sample: Vec<_> = v14::ContractInfoOf::<T>::iter().take(100).collect();
 
 		log::debug!(target: LOG_TARGET, "Taking sample of {} contracts", sample.len());
 
-		let state: Vec<(T::AccountId, old::ContractInfo<T>, BalanceOf<T>, BalanceOf<T>)> = sample
+		let state: Vec<(T::AccountId, v14::ContractInfo<T>, BalanceOf<T>, BalanceOf<T>)> = sample
 			.iter()
 			.map(|(account, contract)| {
 				(
@@ -265,7 +265,7 @@ impl<T: Config> MigrationStep for Migration<T> {
 	#[cfg(feature = "try-runtime")]
 	fn post_upgrade_step(state: Vec<u8>) -> Result<(), TryRuntimeError> {
 		let sample =
-			<Vec<(T::AccountId, old::ContractInfo<T>, BalanceOf<T>, BalanceOf<T>)> as Decode>::decode(
+			<Vec<(T::AccountId, v14::ContractInfo<T>, BalanceOf<T>, BalanceOf<T>)> as Decode>::decode(
 				&mut &state[..],
 			)
 			.expect("pre_upgrade_step provides a valid state; qed");