Make runner protocol documentation publicly available

[jbergstroem]

Seeing how the issue tracking extending platform support by rewriting from .NET was closed as wontfix - I'm opening an issue to track a suggestion made by @davidchisnall:

Make documentation about the protocol runners use available to the public. This way, third party initiatives to extend support to other platforms or rewriting the runner in a more portable language can happen.

[junaruga]

This is an awesome idea!

[dincer81]

Add

[dch]

NB https://github.com/actions/runner/blob/main/docs/design/ already has some info in it - perhaps not enough, but already a good start

[dch]

and https://github.com/ChristopherHX/github-actions-act-runner is already a long way along

[ethomson]

This isn't something that we currently have planned, but I'll put this on our future backlog to take a look at. Thanks for the suggestion!