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/support/src/lib.rs

@@ -1178,7 +1178,7 @@ pub mod pallet_prelude {
 /// # `pallet::storage_version`
 ///
 /// Because the [`pallet::pallet`](#pallet-struct-placeholder-palletpallet-mandatory) macro
-/// implements [`traits::GetStorageVersion`], the current storage version needs to be
+/// implements [`traits::GetStorageVersion`], the in-code storage version needs to be
 /// communicated to the macro. This can be done by using the `pallet::storage_version`
 /// attribute:
 ///
@@ -1190,7 +1190,7 @@ pub mod pallet_prelude {
 /// pub struct Pallet<T>(_);
 /// ```
 ///
-/// If not present, the current storage version is set to the default value.
+/// If not present, the in-code storage version is set to the default value.
 ///
 /// Also see [`pallet::storage_version`](`frame_support::pallet_macros::storage_version`)
 ///