Generate action READMEs from the YAML

[jdbaldry]

I don't trust that any of the READMEs' inputs and outputs match the actual action specification.

I think a quick tool that generates the READMEs from a template that can be used for all actions would be better than relying on maintainers and contributors to keep the two in sync. Of course, CI should enforce that the README matches the action. We could even add an additional workflow that regenerates the READMEs on a branch so that it doesn't need to be performed locally.

I understand that READMEs help with discoverability so I'm not sure that it's justified to remove them in the mean time.