add interfaces docs

Author: euler-mab

docs/interfaces.md

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+# EulerSwap interfaces
+
+## **IEulerSwap interface**
+
+### **Overview**
+
+The `IEulerSwap` interface defines the core functionality for executing token swaps, activating the contract, and verifying the swapping curve invariant.
+
+### **Functions**
+
+#### `swap(uint256 amount0Out, uint256 amount1Out, address to, bytes calldata data) external;`
+
+- **description**: Optimistically sends the requested amounts of tokens to the `to` address, invokes `uniswapV2Call` callback on `to` (if `data` was provided), and then verifies that a sufficient amount of tokens were transferred to satisfy the swapping curve invariant.
+
+#### `activate() external;`
+
+- **description**: Approves the vaults to access the EulerSwap instance's tokens, and enables vaults as collateral. Can be invoked by anybody, and is harmless if invoked again. Calling this function is optional: EulerSwap can be activated on the first swap.
+
+#### `verify(uint256 newReserve0, uint256 newReserve1) external view returns (bool);`
+
+- **description**: Function that defines the shape of the swapping curve. Returns true if the specified reserve amounts would be acceptable (i.e., above and to-the-right of the swapping curve).
+
+### **Accessors**
+
+#### `curve() external view returns (bytes32);`
+
+- **description**: Returns the identifier of the swapping curve.
+
+#### `vault0() external view returns (address);`
+
+- **description**: Returns the address of vault 0.
+
+#### `vault1() external view returns (address);`
+
+- **description**: Returns the address of vault 1.
+
+#### `asset0() external view returns (address);`
+
+- **description**: Returns the address of asset 0.
+
+#### `asset1() external view returns (address);`
+
+- **description**: Returns the address of asset 1.
+
+#### `myAccount() external view returns (address);`
+
+- **description**: Returns the address of the account managing EulerSwap.
+
+#### `initialReserve0() external view returns (uint112);`
+
+- **description**: Returns the initial reserve amount of asset 0.
+
+#### `initialReserve1() external view returns (uint112);`
+
+- **description**: Returns the initial reserve amount of asset 1.
+
+#### `feeMultiplier() external view returns (uint256);`
+
+- **description**: Returns the fee multiplier applied to transactions.
+
+#### `getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 status);`
+
+- **description**: Retrieves the current reserve amounts and contract status.
+
+### **Curve accessors**
+
+#### `priceX() external view returns (uint256);`
+
+- **description**: Returns the price of asset X in terms of asset Y.
+
+#### `priceY() external view returns (uint256);`
+
+- **description**: Returns the price of asset Y in terms of asset X.
+
+#### `concentrationX() external view returns (uint256);`
+
+- **description**: Returns the liquidity concentration of asset X.
+
+#### `concentrationY() external view returns (uint256);`
+
+- **description**: Returns the liquidity concentration of asset Y.
+
+---
+
+## **IEulerSwapPeriphery interface**
+
+### **Overview**
+
+The `IEulerSwapPeriphery` interface provides auxiliary functions for quoting token swap amounts before execution.
+
+### **Functions**
+
+#### `quoteExactInput(address eulerSwap, address tokenIn, address tokenOut, uint256 amountIn) external view returns (uint256);`
+
+- **description**: Calculates how much `tokenOut` can be received for `amountIn` of `tokenIn`.
+
+#### `quoteExactOutput(address eulerSwap, address tokenIn, address tokenOut, uint256 amountOut) external view returns (uint256);`
+
+- **description**: Calculates how much `tokenIn` is required to receive `amountOut` of `tokenOut`.
+
+---
+
+## **IUniswapV2Callee interface**
+
+### **Overview**
+
+The `IUniswapV2Callee` interface defines the callback function used for executing swaps on EulerSwap.
+
+### **Functions**
+
+#### `uniswapV2Call(address sender, uint256 amount0, uint256 amount1, bytes calldata data) external;`
+
+- **description**: Callback function invoked by EulerSwap during a swap operation, allowing the contract to perform additional logic.