docs(config, world/upgrade): add proxy support

docs/pages/config.mdx

@@ -121,3 +121,7 @@ The global configuration keys are all optional.
 
   - **`worldsFile`** a `string`: JSON file for the chain to `World` deploy address mapping.
     The default is `./worlds.json`.
+
+  - <a id="upgradeableWorldImplementation" />
+    **`upgradeableWorldImplementation`** a `bool`: Whether the `World` is to be deployed behind a proxy to [enable upgrades](/world/upgrade).
+    The default is `false`.

docs/pages/world/_meta.js

@@ -9,6 +9,7 @@ export default {
   balance: "Balance",
   "account-delegation": "Account Delegation",
   "batch-calls": "Batch Calls",
+  "upgrade": "Upgrading",
   modules: "Modules",
   reference: "Reference",
 };

docs/pages/world/upgrade.mdx

@@ -0,0 +1,95 @@
+import { CollapseCode } from "../../components/CollapseCode";
+import { Callout } from "nextra/components";
+
+# Upgrading worlds
+
+The [tables](./tables) and [`System`s](./systems) can be upgraded without changing the underlying `World` code.
+However, if the `World` was deployed [behind a proxy](/config#upgradeableWorldImplementation) it is also possible to upgrade the `World` itself.
+This will be useful in the future as we further develop MUD, adding features and fixing bugs.
+
+## Are upgrades possible?
+
+As per [ERC-1967](https://eips.ethereum.org/EIPS/eip-1967#logic-contract-address), the storage slot `0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc` stores the address of the actual `World` contract in a proxy.
+So to identify if a `World` is deployed behind a proxy run these commands:
+
+```sh copy
+WORLD_ADDRESS=0xbe6b85dc88f969e45d8d8ae128c5a9c9744d6464
+cast implementation $WORLD_ADDRESS
+```
+
+If the answer is anything other than zero, the `World` is behind a proxy and can be upgraded.
+
+## The actual upgrade
+
+To upgrade the `World` use these steps in a `package/contracts` directory:
+
+1. Edit `.env` to add `WORLD_ADDRESS` with the address displayed by MUD Dev Tools.
+   For example, you might want to use this file:
+
+   <CollapseCode>
+
+   ```sh filename=".env" showLineNumbers copy {12}
+   # This .env file is for demonstration purposes only.
+   #
+   # This should usually be excluded via .gitignore and the env vars attached to
+   # your deployment environment, but we're including this here for ease of local
+   # development. Please do not commit changes to this file!
+   #
+   # Enable debug logs for MUD CLI
+   DEBUG=mud:*
+   #
+   # Anvil default private key:
+   PRIVATE_KEY=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80
+   WORLD_ADDRESS=0xbe6b85dc88f969e45d8d8ae128c5a9c9744d6464
+   ```
+
+   </CollapseCode>
+
+1. Create this script in `scripts/UpgradeWorld.s.sol`.
+
+   <CollapseCode>
+
+   ```solidity filename="UpgradeWorld.s.sol" copy showLineNumbers {18,23}
+   // SPDX-License-Identifier: MIT
+   pragma solidity >=0.8.24;
+
+   import { Script } from "forge-std/Script.sol";
+   import { console } from "forge-std/console.sol";
+
+   import { World } from "@latticexyz/world/src/World.sol";
+   import { WorldProxy } from "@latticexyz/world/src/WorldProxy.sol";
+
+   contract SetImplementation is Script {
+     function run() external {
+       uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
+       vm.startBroadcast(deployerPrivateKey);
+
+       address proxyAddress = vm.envAddress("WORLD_ADDRESS");
+
+       // Deploy new world implementation
+       World newWorld = new World();
+
+       console.log("proxy:", proxyAddress);
+       console.log("new World:", address(newWorld));
+
+       WorldProxy(payable(proxyAddress)).setImplementation(address(newWorld));
+
+       vm.stopBroadcast();
+     }
+   }
+   ```
+
+   </CollapseCode>
+
+1. Run the script.
+
+   ```sh copy
+   forge script script/UpgradeWorld.s.sol --rpc-url http://localhost:8545 --broadcast
+   ```
+
+1. See the new address.
+
+   ```sh copy
+   source .env
+   cast implementation $WORLD_ADDRESS
+   ```