Botframework-composer: docs needed: how to build custom actions in an ejected runtime

[benbrown]

Is your feature request related to a problem? Please describe.

Our documentation only covers how to turn on the included sample action, and does not cover the process of developing the C# class that implements the action or how these work.

Describe the solution you'd like

We need to have a clear document about how to build the custom action class and how it works with the schema loading system.

[benbrown]

These docs from the SDK are relevant:

https://docs.microsoft.com/en-us/azure/bot-service/bot-builder-dialogs-declarative?view=azure-bot-service-4.0&tabs=csharp#creating-the-schema-file

[zxyanliu]

Updated here.

[benbrown]

Thanks for the great work on this, @zxyanliu!!

[benbrown]

The updated docs can be found here:
https://github.com/MicrosoftDocs/composer-docs-pr/blob/master/composer-docs/how-to-add-custom-action.md

[zxyanliu]

The content is live.
https://docs.microsoft.com/en-us/composer/how-to-add-custom-action