invoke.mdx: stub examples for source actor logic

Author: audionerd

docs/invoke.mdx

@@ -546,22 +546,52 @@ actor.subscribe({
 
 ## Invoking Callbacks
 
+```ts
+invoke: {
+  src: fromCallback(/* … */)
+}
+```
+
 _Coming soon_
 
 ## Invoking Observables
 
+```ts
+invoke: {
+  src: fromObservable(/* … */)
+}
+```
+
 _Coming soon_
 
 ## Invoking Event Observables
 
+```ts
+invoke: {
+  src: fromEventObservable(/* … */)
+}
+```
+
 _Coming soon_
 
 ## Invoking Transitions
 
+```ts
+invoke: {
+  src: fromTransition(/* … */)
+}
+```
+
 _Coming soon_
 
 ## Invoking Machines
 
+```ts
+invoke: {
+  src: createMachine(/* … */)
+}
+```
+
 _Coming soon_
 
 ## Sending Responses