Add XCM documentation to the sdk docs (#2628)

I added a small documentation for XCM in the context of the polkadot SDK

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: command-bot <>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

Cargo.lock

@@ -13312,6 +13312,7 @@ dependencies = [
  "staging-chain-spec-builder",
  "staging-node-cli",
  "staging-parachain-info",
+ "staging-xcm",
  "subkey",
  "substrate-wasm-builder",
 ]

docs/sdk/Cargo.toml

@@ -61,6 +61,9 @@ sp-core = { path = "../../substrate/primitives/core" }
 sp-keyring = { path = "../../substrate/primitives/keyring" }
 sp-runtime = { path = "../../substrate/primitives/runtime" }
 
+# XCM
+xcm = { package = "staging-xcm", path = "../../polkadot/xcm" }
+
 [dev-dependencies]
 parity-scale-codec = "3.6.5"
 scale-info = "2.9.0"

docs/sdk/src/polkadot_sdk/xcm.rs

 //! # XCM
 //!
-//! @KiChjang @franciscoaguirre
-//! TODO: RFCs, xcm-spec, the future of the repo, minimal example perhaps, forward to where actual
-//! docs are hosted. <https://github.com/paritytech/polkadot-sdk-docs/issues/58>
+//! XCM, or Cross-Consensus Messaging, is a **language** to communicate **intentions** between
+//! **consensus systems**.
+//!
+//! ## Overview
+//!
+//! XCM is a standard, whose specification lives in the [xcm format repo](https://github.com/paritytech/xcm-format).
+//! It's agnostic both in programming language and blockchain platform, which means it could be used
+//! in Rust in Polkadot, or in Go or C++ in any other platform like Cosmos or Ethereum.
+//!
+//! It enables different consensus systems to communicate with each other in an expressive manner.
+//! Consensus systems include blockchains, smart contracts, and any other state machine that
+//! achieves consensus in some way.
+//!
+//! XCM is executed on a virtual machine called the XCVM.
+//! Scripts can be written with the XCM language, which are often called XCMs, messages or XCM
+//! programs. Each program is a series of instructions, which get executed one after the other by
+//! the virtual machine. These instructions aim to encompass all major things users typically do in
+//! consensus systems. There are instructions on asset transferring, teleporting, locking, among
+//! others. New instructions are added and changes to the XCVM are made via the [RFC process](https://github.com/paritytech/xcm-format/blob/master/proposals/0032-process.md).
+//!
+//! ## In Polkadot SDK
+//!
+//! The Polkadot SDK allows for easily deploying sovereign blockchains from scratch, all very
+//! customizable. Dealing with many heterogeneous blockchains can be cumbersome.
+//! XCM allows all these blockchains to communicate with an agreed-upon language.
+//! As long as an implementation of the XCVM is implemented, the same XCM program can be executed in
+//! all blockchains and perform the same task.
+//!
+//! ## Implementation
+//!
+//! A ready-to-use Rust implementation lives in the [polkadot-sdk repo](https://github.com/paritytech/polkadot-sdk/tree/master/polkadot/xcm),
+//! but will be moved to its own repo in the future.
+//!
+//! Its main components are:
+//! - `src`: the definition of the basic types and instructions
+//! - [`xcm-executor`](https://paritytech.github.io/polkadot-sdk/master/staging_xcm_executor/struct.XcmExecutor.html):
+//!   an implementation of the virtual machine to execute instructions
+//! - `pallet-xcm`: A FRAME pallet for interacting with the executor
+//! - `xcm-builder`: a collection of types to configure the executor
+//! - `xcm-simulator`: a playground for trying out different XCM programs and executor
+//!   configurations
+//!
+//! ## Example
+//!
+//! To perform the very usual operation of transferring assets, the following XCM program can be
+//! used:
+#![doc = docify::embed!("src/polkadot_sdk/xcm.rs", example_transfer)]
+//!
+//! ## Get started
+//!
+//! To learn how it works and to get started, go to the [XCM docs](https://paritytech.github.io/xcm-docs/).
+
+#[cfg(test)]
+mod tests {
+	use xcm::latest::prelude::*;
+
+	#[docify::export]
+	#[test]
+	fn example_transfer() {
+		let _transfer_program = Xcm::<()>(vec![
+			WithdrawAsset((Here, 100u128).into()),
+			BuyExecution { fees: (Here, 100u128).into(), weight_limit: Unlimited },
+			DepositAsset {
+				assets: All.into(),
+				beneficiary: AccountId32 { id: [0u8; 32].into(), network: None }.into(),
+			},
+		]);
+	}
+}