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/v14.rs

@@ -44,7 +44,7 @@ use sp_runtime::{traits::Zero, Saturating};
 #[cfg(feature = "try-runtime")]
 use sp_std::collections::btree_map::BTreeMap;
 
-mod old {
+mod v13 {
 	use super::*;
 
 	pub type BalanceOf<T, OldCurrency> = <OldCurrency as frame_support::traits::Currency<
@@ -61,7 +61,7 @@ mod old {
 	{
 		pub owner: AccountIdOf<T>,
 		#[codec(compact)]
-		pub deposit: old::BalanceOf<T, OldCurrency>,
+		pub deposit: v13::BalanceOf<T, OldCurrency>,
 		#[codec(compact)]
 		pub refcount: u64,
 		pub determinism: Determinism,
@@ -86,14 +86,14 @@ where
 	let code = vec![42u8; len as usize];
 	let hash = T::Hashing::hash(&code);
 
-	let info = old::CodeInfo {
+	let info = v13::CodeInfo {
 		owner: account,
 		deposit: 10_000u32.into(),
 		refcount: u64::MAX,
 		determinism: Determinism::Enforced,
 		code_len: len,
 	};
-	old::CodeInfoOf::<T, OldCurrency>::insert(hash, info);
+	v13::CodeInfoOf::<T, OldCurrency>::insert(hash, info);
 }
 
 #[cfg(feature = "try-runtime")]
@@ -105,9 +105,9 @@ where
 	OldCurrency: ReservableCurrency<<T as frame_system::Config>::AccountId>,
 {
 	/// Total reserved balance as code upload deposit for the owner.
-	reserved: old::BalanceOf<T, OldCurrency>,
+	reserved: v13::BalanceOf<T, OldCurrency>,
 	/// Total balance of the owner.
-	total: old::BalanceOf<T, OldCurrency>,
+	total: v13::BalanceOf<T, OldCurrency>,
 }
 
 #[derive(Encode, Decode, MaxEncodedLen, DefaultNoBound)]
@@ -134,11 +134,11 @@ where
 
 	fn step(&mut self) -> (IsFinished, Weight) {
 		let mut iter = if let Some(last_hash) = self.last_code_hash.take() {
-			old::CodeInfoOf::<T, OldCurrency>::iter_from(
-				old::CodeInfoOf::<T, OldCurrency>::hashed_key_for(last_hash),
+			v13::CodeInfoOf::<T, OldCurrency>::iter_from(
+				v13::CodeInfoOf::<T, OldCurrency>::hashed_key_for(last_hash),
 			)
 		} else {
-			old::CodeInfoOf::<T, OldCurrency>::iter()
+			v13::CodeInfoOf::<T, OldCurrency>::iter()
 		};
 
 		if let Some((hash, code_info)) = iter.next() {
@@ -194,7 +194,7 @@ where
 
 	#[cfg(feature = "try-runtime")]
 	fn pre_upgrade_step() -> Result<Vec<u8>, TryRuntimeError> {
-		let info: Vec<_> = old::CodeInfoOf::<T, OldCurrency>::iter().collect();
+		let info: Vec<_> = v13::CodeInfoOf::<T, OldCurrency>::iter().collect();
 
 		let mut owner_balance_allocation =
 			BTreeMap::<AccountIdOf<T>, BalanceAllocation<T, OldCurrency>>::new();