How are you using this web site? The entire thing is "discoverable".

What you're talking about is how can you publish a machine readable discoverable API. Just like RPC, there is no "well known" endpoint for getting the API specification.

The RPC IDL is effectively the same as "go read the docs". How is downloading a "formal published IDL" any different to a "formal published OpenAPI specification"?

gRPC just has an entire infrastructure of compilers, parsers and language libraries that generate stub code that you then have to go and "fill in".

OpenAPI is a pretty good standard for defining an HTTP based REST API.