docs: remove require, add example gno doc (#3576)

This pull request is for :

- remove `require` statement from official documentation
[#3123](#3123)
- add an example that was `//TODO` in the `effective-gno / Documentation
is for users` section

docs/concepts/effective-gno.md

@@ -297,7 +297,57 @@ main purpose in Gno is for discoverability. This shift towards user-centric
 documentation reflects the broader shift in Gno towards making code more
 accessible and understandable for all users, not just developers.
 
-TODO: `func ExampleXXX`.
+Here's an example from [grc20](https://gno.land/p/demo/grc/grc20$source&file=types.gno)
+to illustrate the concept:
+
+```go
+// Teller interface defines the methods that a GRC20 token must implement. It
+// extends the TokenMetadata interface to include methods for managing token
+// transfers, allowances, and querying balances.
+//
+// The Teller interface is designed to ensure that any token adhering to this
+// standard provides a consistent API for interacting with fungible tokens.
+type Teller interface {
+	exts.TokenMetadata
+
+	// Returns the amount of tokens in existence.
+	TotalSupply() uint64
+
+	// Returns the amount of tokens owned by `account`.
+	BalanceOf(account std.Address) uint64
+
+	// Moves `amount` tokens from the caller's account to `to`.
+	//
+	// Returns an error if the operation failed.
+	Transfer(to std.Address, amount uint64) error
+
+	// Returns the remaining number of tokens that `spender` will be
+	// allowed to spend on behalf of `owner` through {transferFrom}. This is
+	// zero by default.
+	//
+	// This value changes when {approve} or {transferFrom} are called.
+	Allowance(owner, spender std.Address) uint64
+
+	// Sets `amount` as the allowance of `spender` over the caller's tokens.
+	//
+	// Returns an error if the operation failed.
+	//
+	// IMPORTANT: Beware that changing an allowance with this method brings
+	// the risk that someone may use both the old and the new allowance by
+	// unfortunate transaction ordering. One possible solution to mitigate
+	// this race condition is to first reduce the spender's allowance to 0
+	// and set the desired value afterwards:
+	// https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+	Approve(spender std.Address, amount uint64) error
+
+	// Moves `amount` tokens from `from` to `to` using the
+	// allowance mechanism. `amount` is then deducted from the caller's
+	// allowance.
+	//
+	// Returns an error if the operation failed.
+	TransferFrom(from, to std.Address, amount uint64) error
+}
+```
 
 ### Reflection is never clear
 

docs/concepts/gno-modules.md

@@ -24,20 +24,14 @@ The gno command-line tool provides several commands to work with the gno.mod fil
 
 - **gno mod init**: small helper to initialize a new `gno.mod` file.
 - **gno mod download**: downloads the dependencies specified in the gno.mod file. This command fetches the required dependencies from chain and ensures they are available for local testing and development.
-- **gno mod tidy**: removes any unused dependency and adds any required but not yet listed in the file -- most of the maintenance you'll usually need to do!
 - **gno mod why**: explains why the specified package or module is being kept by `gno mod tidy`.
 
 ## Sample `gno.mod` file
 
 ```
 module gno.land/p/demo/sample
 
-require (
-    gno.land/p/demo/avl v0.0.0-latest
-    gno.land/p/demo/testutils v0.0.0-latest
-)
-
 ```
 
 - **`module gno.land/p/demo/sample`**: specifies the package/realm import path.
-- **`require` Block**: lists the required dependencies. Here using the latest available versions of "gno.land/p/demo/avl" and "gno.land/p/demo/testutils". These dependencies should be specified with the version "v0.0.0-latest" since on-chain packages currently do not support versioning.
+