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>

Cargo.lock

@@ -9965,6 +9965,26 @@ dependencies = [
  "sp-std 14.0.0",
 ]
 
+[[package]]
+name = "pallet-example-single-block-migrations"
+version = "0.0.1"
+dependencies = [
+ "docify",
+ "frame-executive",
+ "frame-support",
+ "frame-system",
+ "frame-try-runtime",
+ "log",
+ "pallet-balances",
+ "parity-scale-codec",
+ "scale-info",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std 14.0.0",
+ "sp-version",
+]
+
 [[package]]
 name = "pallet-example-split"
 version = "10.0.0"
@@ -10006,6 +10026,7 @@ dependencies = [
  "pallet-example-frame-crate",
  "pallet-example-kitchensink",
  "pallet-example-offchain-worker",
+ "pallet-example-single-block-migrations",
  "pallet-example-split",
  "pallet-example-tasks",
 ]
@@ -13385,6 +13406,8 @@ dependencies = [
  "cumulus-pallet-parachain-system",
  "docify",
  "frame",
+ "frame-executive",
+ "frame-support",
  "frame-system",
  "kitchensink-runtime",
  "pallet-aura",
@@ -13393,9 +13416,11 @@ dependencies = [
  "pallet-collective",
  "pallet-default-config-example",
  "pallet-democracy",
+ "pallet-example-single-block-migrations",
  "pallet-examples",
  "pallet-multisig",
  "pallet-proxy",
+ "pallet-scheduler",
  "pallet-timestamp",
  "pallet-transaction-payment",
  "pallet-utility",
@@ -13418,6 +13443,7 @@ dependencies = [
  "sp-io",
  "sp-keyring",
  "sp-runtime",
+ "sp-version",
  "staging-chain-spec-builder",
  "staging-node-cli",
  "staging-parachain-info",