Add synced actions docs

docs/content/Development/MUI2/Syncing/Synced-Actions.md

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+# Synced Actions
+
+```java
+public class MuiTestMachine extends MetaMachine implements IMuiMachine {
+
+    public MuiTestMachine(BlockEntityCreationInfo info) {
+        super(info);
+    }
+
+    public int number = 0;
+
+    @Override
+    public ModularPanel buildUI(PosGuiData data, PanelSyncManager syncManager, UISettings settings) {
+        var panel = GTGuis.createPanel(this, 176, 168);
+
+        var numberSyncValue = new IntSyncValue(() -> this.number, (newValue) -> this.number = newValue);
+        syncManager.syncValue("number", numberSyncValue);
+
+        syncManager.registerServerSyncedAction("randomButtonPressed", (packet) -> {
+            this.number = getLevel().getRandom().nextInt();
+        });
+
+        Column contents = new Column();
+        contents.child(IKey.dynamic(() -> Component.literal("Number: " + number))
+                .asWidget()
+                .width(100)
+                .height(16)
+                .margin(4));
+
+        contents.child(new ButtonWidget<>()
+                .onMousePressed((x, y, button) -> {
+                    if (button == 0) {
+                        syncManager.callSyncedAction("randomButtonPressed");
+                    }
+                    return true;
+                })
+                .size(16));
+        return panel.child(contents);
+    }
+}
+
+```
+
+Synced actions are useful when you need a client-side input (e.g. a button press) to trigger an action on the server.  
+
+In this case we create a synced action called `randomButtonPressed`, and when the button is pressed on the client, the `randomButtonPressed` signal is sent to the server and the relevant action is executed.
+
+If you need to pass other data with your action, you can pass a `(packet) -> {...}` as second argument to `callSyncedAction` and serialize your data there, and then deserialize it from the `packet` argument in the synced action definition.