Update Solidity files in docs (#4956)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

contracts/mocks/docs/token/ERC20/GLDToken.sol

@@ -0,0 +1,11 @@
+// contracts/GLDToken.sol
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {ERC20} from "../../../../token/ERC20/ERC20.sol";
+
+contract GLDToken is ERC20 {
+    constructor(uint256 initialSupply) ERC20("Gold", "GLD") {
+        _mint(msg.sender, initialSupply);
+    }
+}