Add mouse buttons guide

website/docs/guides/mouse-buttons.mdx

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+# Mouse Buttons
+
+The Wails runtime intercepts mouse clicks to determine whether a frameless window needs resizing or a window needs to be moved.
+It has been asked how to detect when a mouse click has occurred. The following code shows how to detect mouse clicks:
+
+```javascript
+
+window.addEventListener('mousedown', handleMouseButtonDown);
+
+function handleMouseButtonDown(event) {
+  if (event.button === 0) {
+    // left mouse button
+  } else if (event.button === 1) {
+    // middle mouse button
+  } else if (event.button === 2) {
+    // right mouse button
+  } else if (event.button === 3) {
+    // back mouse button
+  } else if (event.button === 4) {
+    // forward mouse button
+  } else {
+    // other mouse button
+  }
+}
+
+```
+Reference: https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/button