docs(world): add modules to new docs (#1856)

next-docs/pages/world/modules.mdx

@@ -1 +1,38 @@
 # Modules
+
+Modules are onchain installation scripts that create resources and their associated configuration when called by a `World`.
+This is somewhat similar to one of the use cases for [foundry scripts](https://book.getfoundry.sh/tutorials/solidity-scripting), except that modules are deployed onchain and can be used by any `World` on the same chain.
+
+## Module installation
+
+Modules can be installed using [`World.installModule(address moduleAddress, bytes memory initData)`](https://github.com/latticexyz/mud/blob/main/packages/world/src/modules/core/implementations/ModuleInstallationSystem.sol#L17-L37).
+When you do this, the `World` calls the module's `install` function with `initData` as the argument(s).
+Because this is a call, the module does not have any administrative permissions on the `World`.
+
+Alternatively, the owner of the root namespace can install modules using [`World.installRootModule(address moduleAddress, bytes memory initData)`](https://github.com/latticexyz/mud/blob/main/packages/world/src/World.sol#L90-L119).
+In this case, the `World` uses delegatecall and module has all the permissions of a `System` in the root namespace.
+
+## Writing modules
+
+The common use for a module is to add functionality to a `World`.
+In most cases we expect that a module would:
+
+1. Create a [namespace](./namespaces-access-control.mdx) for the new functionality.
+1. Create the [tables](./.tables) and [`System`s](./systems) for the new functionality.
+1. Create any access permissions required (beyond the default, which is that a `System` has access to its own namespace).
+1. Either assign the ownership of the new namespace to an entity that would administer it (a user, a multisig, etc.) or burn it by assigning the namespace ownership to `address(0)`.
+
+Root modules run with the same permission as `System`s in the root namespace, so they do not need to create a namespace for themselves.
+They can just create their own tables and `System`s as needed.
+
+## Sample modules
+
+MUD comes with several modules you can use or inspect to help you write your own:
+
+- [`KeysInTable`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/keysintable) - automatically tracks the keys in a table to make them enumerable.
+- [`KeysWithValue`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/keyswithvalue) - automatically tracks a reverse mapping for a table that maps a value hash to a list of keys with this value.
+- [`Puppet`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/puppet) - installs the PuppetDelegationControl to allow creating Puppet contracts, eg used by the ERC20 and ERC721 modules.
+- [`ERC20`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/erc20-puppet) - installs an ERC20 token into a namespace in a World.
+- [`ERC721`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/erc721-puppet) - installs an ERC721 token into a namespace in a World.
+- [`UniqueEntity`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/uniqueentity) - add methods to get a unique entity ID.
+- [`Standard Delegations`](https://github.com/latticexyz/mud/tree/main/packages/world-modules/src/modules/std-delegations) - add delegations limited by time or number of calls.