chore: add IPC docs

ipc.md

@@ -0,0 +1,76 @@
+# Context Menu IPC Integration
+
+If you want to display custom menu items in the chat context menu, you can use
+Chat 2's IPC.
+
+Here's an example.
+
+```c#
+public class ContextMenuIntegration {
+    // This is used to register your plugin with the IPC. It will return an ID
+    // that you should save for later.
+    private ICallGateSubscriber<string> Register { get; }
+    // This is used to unregister your plugin from the IPC. You should call this
+    // when your plugin is unloaded.
+    private ICallGateSubscriber<string, object?> Unregister { get; }
+    // You should subscribe to this event in order to receive a notification
+    // when Chat 2 is loaded or updated, so you can re-register.
+    private ICallGateSubscriber<object?> Available { get; }
+    // Subscribe to this to draw your custom context menu items.
+    private ICallGateSubscriber<string, PlayerPayload?, ulong, Payload?, SeString?, SeString?, object?> Invoke { get; }
+
+    // The registration ID.
+    private string? _id;
+
+    public ChatTwoIpc(DalamudPluginInterface @interface) {
+        this.Register = @interface.GetIpcSubscriber<string>("ChatTwo.Register");
+        this.Unregister = @interface.GetIpcSubscriber<string, object?>("ChatTwo.Unregister");
+        this.Invoke = @interface.GetIpcSubscriber<string, PlayerPayload?, ulong, Payload?, SeString?, SeString?, object?>("ChatTwo.Invoke");
+        this.Available = @interface.GetIpcSubscriber<object?>("ChatTwo.Available");
+    }
+
+    public void Enable() {
+        // When Chat 2 becomes available (if it loads after this plugin) or when
+        // Chat 2 is updated, register automatically.
+        this.Available.Subscribe(() => this.Register());
+        // Register if Chat 2 is already loaded.
+        this.Register();
+        
+        // Listen for context menu events.
+        this.Invoke.Subscribe(this.Integration);
+    }
+    
+    private void Register() {
+        // Register and save the registration ID.
+        this._id = this.Register.InvokeFunc();
+    }
+
+    public void Disable() {
+        if (this._id != null) {
+            this.Unregister.InvokeAction(this._id);
+            this._id = null;
+        }
+
+        this.Invoke.Unsubscribe(this.Integration);
+    }
+
+    private void Integration(string id, PlayerPayload? sender, ulong contentId, Payload? payload, SeString? senderString, SeString? content) {
+        // Make sure the ID is the same as the saved registration ID.
+        if (id != this._id) {
+            return;
+        }
+
+        // Draw your custom menu items here.
+        // sender is the first PlayerPayload contained in the sender SeString
+        // contentId is the content ID of the message sender or 0 if not known
+        // payload is the payload that was right-clicked, if any (excluding text)
+        // senderString is the message sender SeString
+        // content is the message content SeString
+        if (ImGui.Selectable("Test plugin")) {
+            PluginLog.Log($"hi!\nsender: {sender}\ncontent id: {contentId:X}\npayload: {payload}\nsender string: {senderString}\ncontent string: {content}");
+        }
+    }
+}
+```
+
+All integrations are called inside of an ImGui `BeginMenu`.