Add documentation around FRAME Origin (paritytech#3362)

Does the following: 

- Add a reference doc page named `frame_runtime_types`, which explains
what types like `RuntimeOrigin`, `RuntimeCall` etc are.
- On top of it, it adds a reference doc page called `frame_origin` which
explains a few important patterns that we use around origins
- And finally brushes up `#[frame::origin]` docs. 
- Updates the theme, sidebar and favicon to look like: 

<img width="1728" alt="Screenshot 2024-02-20 at 12 16 00"
src="https://github.com/paritytech/polkadot-sdk/assets/5588131/6d60a16b-2081-411b-8869-43b91920cca9">


All of this was inspired by
https://substrate.stackexchange.com/questions/10992/how-do-you-find-the-public-key-for-the-medium-spender-track-origin/10993

closes paritytech/polkadot-sdk-docs#45
closes paritytech/polkadot-sdk-docs#43
contributes / overlaps with
paritytech#2638 cc @liamaharon
deprecation companion:
polkadot-developers/substrate-docs#2131
pba-content companion:
Polkadot-Blockchain-Academy/pba-content#977

---------

Co-authored-by: Radha <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>

substrate/frame/src/lib.rs

@@ -163,6 +163,12 @@ pub mod runtime {
 			ConstU32, ConstU64, ConstU8,
 		};
 
+		/// Primary types used to parameterize `EnsureOrigin` and `EnsureRootWithArg`.
+		pub use frame_system::{
+			EnsureNever, EnsureNone, EnsureRoot, EnsureRootWithSuccess, EnsureSigned,
+			EnsureSignedBy,
+		};
+
 		/// Types to define your runtime version.
 		pub use sp_version::{create_runtime_str, runtime_version, RuntimeVersion};
 

substrate/frame/support/procedural/src/construct_runtime/expand/origin.rs

@@ -104,7 +104,7 @@ pub fn expand_outer_origin(
 		#[doc = #doc_string]
 		#[derive(Clone)]
 		pub struct RuntimeOrigin {
-			caller: OriginCaller,
+			pub caller: OriginCaller,
 			filter: #scrate::__private::sp_std::rc::Rc<Box<dyn Fn(&<#runtime as #system_path::Config>::RuntimeCall) -> bool>>,
 		}
 

substrate/frame/support/procedural/src/lib.rs

@@ -1480,22 +1480,11 @@ pub fn validate_unsigned(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()
 }
 
-/// The `#[pallet::origin]` attribute allows you to define some origin for the pallet.
 ///
-/// Item must be either a type alias, an enum, or a struct. It needs to be public.
-///
-/// E.g.:
-///
-/// ```ignore
-/// #[pallet::origin]
-/// pub struct Origin<T>(PhantomData<(T)>);
-/// ```
-///
-/// **WARNING**: modifying origin changes the outer runtime origin. This outer runtime origin
-/// can be stored on-chain (e.g. in `pallet-scheduler`), thus any change must be done with care
-/// as it might require some migration.
+/// ---
 ///
-/// NOTE: for instantiable pallets, the origin must be generic over `T` and `I`.
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::origin`.
 #[proc_macro_attribute]
 pub fn origin(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()

substrate/frame/support/src/lib.rs

@@ -2274,9 +2274,8 @@ pub mod pallet_macros {
 	pub use frame_support_procedural::{
 		composite_enum, config, disable_frame_system_supertrait_check, error, event,
 		extra_constants, feeless_if, generate_deposit, generate_store, getter, hooks,
-		import_section, inherent, no_default, no_default_bounds, origin, pallet_section,
-		storage_prefix, storage_version, type_value, unbounded, validate_unsigned, weight,
-		whitelist_storage,
+		import_section, inherent, no_default, no_default_bounds, pallet_section, storage_prefix,
+		storage_version, type_value, unbounded, validate_unsigned, weight, whitelist_storage,
 	};
 
 	/// Allows a pallet to declare a set of functions as a *dispatchable extrinsic*. In
@@ -2718,29 +2717,29 @@ pub mod pallet_macros {
 	/// }
 	/// ```
 	pub use frame_support_procedural::storage;
-	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// This attribute is attached to a function inside an `impl` block annotated with
 	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define the conditions for a
 	/// given work item to be valid.
 	///
 	/// It takes a closure as input, which is then used to define the condition. The closure
 	/// should have the same signature as the function it is attached to, except that it should
 	/// return a `bool` instead.
 	pub use frame_support_procedural::task_condition;
-	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// This attribute is attached to a function inside an `impl` block annotated with
 	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define the index of a given
 	/// work item.
 	///
 	/// It takes an integer literal as input, which is then used to define the index. This
 	/// index should be unique for each function in the `impl` block.
 	pub use frame_support_procedural::task_index;
-	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// This attribute is attached to a function inside an `impl` block annotated with
 	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define an iterator over the
 	/// available work items for a task.
 	///
 	/// It takes an iterator as input that yields a tuple with same types as the function
 	/// arguments.
 	pub use frame_support_procedural::task_list;
-	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// This attribute is attached to a function inside an `impl` block annotated with
 	/// [`pallet::tasks_experimental`](`tasks_experimental`) define the weight of a given work
 	/// item.
 	///
@@ -2773,6 +2772,61 @@ pub mod pallet_macros {
 	/// Now, this can be executed as follows:
 	#[doc = docify::embed!("src/tests/tasks.rs", tasks_work)]
 	pub use frame_support_procedural::tasks_experimental;
+
+	/// Allows a pallet to declare a type as an origin.
+	///
+	/// If defined as such, this type will be amalgamated at the runtime level into
+	/// `RuntimeOrigin`, very similar to [`call`], [`error`] and [`event`]. See
+	/// [`composite_enum`] for similar cases.
+	///
+	/// Origin is a complex FRAME topics and is further explained in `polkadot_sdk_docs`.
+	///
+	/// ## Syntax Variants
+	///
+	/// ```
+	/// #[frame_support::pallet]
+	/// mod pallet {
+	///     # use frame_support::pallet_prelude::*;
+	///     # #[pallet::config]
+	///     # pub trait Config: frame_system::Config {}
+	///     # #[pallet::pallet]
+	///     # pub struct Pallet<T>(_);
+	/// 	/// On the spot declaration.
+	///     #[pallet::origin]
+	/// 	#[derive(PartialEq, Eq, Clone, RuntimeDebug, Encode, Decode, TypeInfo, MaxEncodedLen)]
+	/// 	pub enum Origin {
+	/// 		Foo,
+	/// 		Bar,
+	/// 	}
+	/// }
+	/// ```
+	///
+	/// Or, more commonly used:/
+	///
+	/// ```
+	/// #[frame_support::pallet]
+	/// mod pallet {
+	///     # use frame_support::pallet_prelude::*;
+	///     # #[pallet::config]
+	///     # pub trait Config: frame_system::Config {}
+	///     # #[pallet::pallet]
+	///     # pub struct Pallet<T>(_);
+	/// 	#[derive(PartialEq, Eq, Clone, RuntimeDebug, Encode, Decode, TypeInfo, MaxEncodedLen)]
+	/// 	pub enum RawOrigin {
+	/// 		Foo,
+	/// 		Bar,
+	/// 	}
+	///
+	/// 	#[pallet::origin]
+	/// 	pub type Origin = RawOrigin;
+	/// }
+	/// ```
+	///
+	/// ## Warning
+	///
+	/// Modifying any pallet's origin type will cause the runtime level origin type to also
+	/// change in encoding. If stored anywhere on-chain, this will require a data migration.
+	pub use frame_support_procedural::origin;
 }
 
 #[deprecated(note = "Will be removed after July 2023; Use `sp_runtime::traits` directly instead.")]

substrate/frame/system/src/lib.rs

@@ -452,6 +452,7 @@ pub mod pallet {
 			+ Clone
 			+ OriginTrait<Call = Self::RuntimeCall, AccountId = Self::AccountId>;
 
+		#[docify::export(system_runtime_call)]
 		/// The aggregated `RuntimeCall` type.
 		#[pallet::no_default_bounds]
 		type RuntimeCall: Parameter