Update docs

contracts/mocks/RegressionImplementation.sol

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../proxy/utils/Initializable.sol";
+
+contract Implementation1 is Initializable {
+    uint256 internal _value;
+
+    function initialize() public initializer {}
+
+    function setValue(uint256 _number) public {
+        _value = _number;
+    }
+}
+
+contract Implementation2 is Initializable {
+    uint256 internal _value;
+
+    function initialize() public initializer {}
+
+    function setValue(uint256 _number) public {
+        _value = _number;
+    }
+
+    function getValue() public view returns (uint256) {
+        return _value;
+    }
+}
+
+contract Implementation3 is Initializable {
+    uint256 internal _value;
+
+    function initialize() public initializer {}
+
+    function setValue(uint256 _number) public {
+        _value = _number;
+    }
+
+    function getValue(uint256 _number) public view returns (uint256) {
+        return _value + _number;
+    }
+}
+
+contract Implementation4 is Initializable {
+    uint256 internal _value;
+
+    function initialize() public initializer {}
+
+    function setValue(uint256 _number) public {
+        _value = _number;
+    }
+
+    function getValue() public view returns (uint256) {
+        return _value;
+    }
+
+    fallback() external {
+        _value = 1;
+    }
+}