Remove DOMAIN_SEPARATOR from Votes and update docs examples (#4297)

Co-authored-by: Qiwei Yang <yangqiwei97@gmail.com>
Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

contracts/mocks/docs/governance/MyToken.sol

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.19;
+
+import "../../../token/ERC20/ERC20.sol";
+import "../../../token/ERC20/extensions/ERC20Permit.sol";
+import "../../../token/ERC20/extensions/ERC20Votes.sol";
+
+contract MyToken is ERC20, ERC20Permit, ERC20Votes {
+    constructor() ERC20("MyToken", "MTK") ERC20Permit("MyToken") {}
+
+    // The functions below are overrides required by Solidity.
+
+    function _update(address from, address to, uint256 amount) internal override(ERC20, ERC20Votes) {
+        super._update(from, to, amount);
+    }
+
+    function nonces(address owner) public view virtual override(ERC20Permit, Nonces) returns (uint256) {
+        return super.nonces(owner);
+    }
+}