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/v09.rs

@@ -28,7 +28,7 @@ use frame_support::{pallet_prelude::*, storage_alias, DefaultNoBound, Identity};
 use sp_runtime::TryRuntimeError;
 use sp_std::prelude::*;
 
-mod old {
+mod v8 {
 	use super::*;
 
 	#[derive(Encode, Decode)]
@@ -50,14 +50,14 @@ mod old {
 #[cfg(feature = "runtime-benchmarks")]
 pub fn store_old_dummy_code<T: Config>(len: usize) {
 	use sp_runtime::traits::Hash;
-	let module = old::PrefabWasmModule {
+	let module = v8::PrefabWasmModule {
 		instruction_weights_version: 0,
 		initial: 0,
 		maximum: 0,
 		code: vec![42u8; len],
 	};
 	let hash = T::Hashing::hash(&module.code);
-	old::CodeStorage::<T>::insert(hash, module);
+	v8::CodeStorage::<T>::insert(hash, module);
 }
 
 #[derive(Encode, Decode)]
@@ -89,9 +89,9 @@ impl<T: Config> MigrationStep for Migration<T> {
 
 	fn step(&mut self) -> (IsFinished, Weight) {
 		let mut iter = if let Some(last_key) = self.last_code_hash.take() {
-			old::CodeStorage::<T>::iter_from(old::CodeStorage::<T>::hashed_key_for(last_key))
+			v8::CodeStorage::<T>::iter_from(v8::CodeStorage::<T>::hashed_key_for(last_key))
 		} else {
-			old::CodeStorage::<T>::iter()
+			v8::CodeStorage::<T>::iter()
 		};
 
 		if let Some((key, old)) = iter.next() {
@@ -115,7 +115,7 @@ impl<T: Config> MigrationStep for Migration<T> {
 
 	#[cfg(feature = "try-runtime")]
 	fn pre_upgrade_step() -> Result<Vec<u8>, TryRuntimeError> {
-		let sample: Vec<_> = old::CodeStorage::<T>::iter().take(100).collect();
+		let sample: Vec<_> = v8::CodeStorage::<T>::iter().take(100).collect();
 
 		log::debug!(target: LOG_TARGET, "Taking sample of {} contract codes", sample.len());
 		Ok(sample.encode())
@@ -123,7 +123,7 @@ impl<T: Config> MigrationStep for Migration<T> {
 
 	#[cfg(feature = "try-runtime")]
 	fn post_upgrade_step(state: Vec<u8>) -> Result<(), TryRuntimeError> {
-		let sample = <Vec<(CodeHash<T>, old::PrefabWasmModule)> as Decode>::decode(&mut &state[..])
+		let sample = <Vec<(CodeHash<T>, v8::PrefabWasmModule)> as Decode>::decode(&mut &state[..])
 			.expect("pre_upgrade_step provides a valid state; qed");
 
 		log::debug!(target: LOG_TARGET, "Validating sample of {} contract codes", sample.len());