Add basic docs

docs/pages/world/systems.mdx

@@ -227,6 +227,43 @@ If your `System` needs run both from the root namespace and from other namespace
 
    </details>
 
+#### System Libraries
+
+<Callout type="warning" emoji="⚠️">
+  This is an experimental feature and is not recommended for production use yet. Its API is expected to change in the
+  future.
+</Callout>
+
+By setting the `generateSystemLibraries` codegen setting to true in your MUD config, the `worldgen` CLI will automatically generate a Solidity library for each system in your project. This feature simplifies inter-system communication by providing a convenient way of calling functions between systems.
+
+Each generated library defines and exports a custom user-defined type that includes all the functions defined in the corresponding system. Internally, these system libraries utilize the World's `call` and `callFrom` methods to execute the actual system contracts.
+
+```solidity filename="MySystem.sol"
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.8.24;
+
+import { System } from "@latticexyz/world/src/System.sol";
+
+// Library generated for MoveSystem,
+import { moveSystem } from "./codegen/libraries/MoveSystemLib.sol";
+
+contract MySystem is System {
+  function someAction(uint256 x, uint256 y) external {
+    // All functions defined in the MoveSystem system are available globally for the exported `moveSystem` value
+    moveSystem.move(x, y);
+
+    // You can also use delegations (if they exist) by using `callFrom`
+    moveSystem.callFrom(_msgSender()).move(x, y);
+
+    // If MySystem is a root system, you need to use `callAsRoot`
+    moveSystem.callAsRoot().move(x, y);
+
+    // If MySystem is a root system, it can call other systems on behalf of any address with `callAsRootFrom`
+    moveSystem.callAsRootFrom(_msgSender()).move(x, y);
+  }
+}
+```
+
 ## Registering systems
 
 For a `System` to be callable from a `World` it has to be [registered](https://github.com/latticexyz/mud/blob/main/packages/world/src/modules/init/implementations/WorldRegistrationSystem.sol#L115-L178).