I don't know about gRPC specifically, but since you most of the times use Protocol Buffers to define your API, protoc-gen-docbook would be an option. I'm personally using a C# port that I enhanced with our options of
https://code.google.com/archive/p/protoc-gen-docbook/ right now for an older proto2 project. There seems to be an up2date version written in GoLang called
protoc-gen-doc where you can also use a Docker container. I haven't used this one myself, but will look into it soon for my own gRPC API documentation. Since this also can export docbook, it looks a like good replacement while keeping my existing PDF build-chain (using
Apache FOP).