I propose this topic as feature request, but it's also a documentation issue, as lack of details in user guide paragraph: https://rasa.com/docs/rasa/core/actions/#custom-actions.

What specified in paragraph Execute Actions in Other Code is obscure to me, and details at the API documentation link Action Server are also unclear.

Is the endpoint a externalActionServer:port/webhook POST?
I wish I had a detailed description of in/out parameters in API details page. Maybe a pseudo curl example of call to an external sever, with few different use cases (generic custom actions, form actions, NLG actions, using dockling.., etc,) could help a lot.

Besides, I'd also add a simple example of implementation of an external server, maybe to be inserted in https://github.com/RasaHQ/rasa/tree/master/examples, maybe something like the "formbot", realized awith an external action server built in node :)

Note: I guess there is a huge number of developers interested to use RASA but to build actions not built in Python but in other languages (e.g. Java and especially in nodejs), for obvious reasons (connection with legacy sw, previous made backend logics, etc.). I'd do all possible to break barriers for incoming developers in any not-python language, allowing them to enter the splendid RASA ecosystem :)