Document the trusted publisher exchange token API

[segiddins]

No description provided.

[indirect]

Looks good! Where can I see the list of attributes that are expected or required in the JWT?

[segiddins]

I think it's fine to treat it as an opaque string, since it's coming straight from a supported trusted publisher platform (which for now just means github actions).

[indirect]

Since this API is intended to be hit only by OIDC providers, don't we need to document what someone would need to provide to be able to be a provider? Or is there a different point to this API documentation and I'm just misunderstanding the point...

[segiddins]

I had thought you wanted the endpoint documented for those choosing to not use our official action?

[indirect]

Okay, so we have identified two potential audiences for this documentation:

1. End-user who wants to use trusted publishing from GitHub actions without using our pre-built release-gem.
2. Developer who wants to implement a JWT in order to become a provider for RubyGems Trusted Publishing.

For use case 1, we should explain how to get a JWT that this API will accept from GitHub. Does GitHub pre-fill the $ID_TOKEN variable with a JWT that works for trusted publishing? If yes, we should say that. If not, we should explain how to get one of those JWTs.

For use case 2, we should explain what needs to be inside the JWT to make it possible to be a provider. We will likely want to support (for example) GitLab in the future, and we need some document we can link them to to tell them what to do before we can add them as a provider.

Does that work? Am I missing anything?