I would be really interested in being part of this working group!
My background is QA and Quality Management for medical devices so I've written a lot of technical documentation - mostly bug tickets, technical software specification, and documentation of processes like system design and software development lifecycle (workflows etc), and creating and implementing styleguides, but also some more outward facing things like test plans, user guides and user manuals.
I unfortunately don't know the first thing about Sphinx, Swagger, and Vale, but I'm sure there's some overlap with systems I've used before and I'm eager to learn so that I can generally be helpful to this project and similar projects.
Also want to add that this is not just something that I'm experienced in, but I'm passionate about creating good documentation and improving my skills. I like this project, I have some free time right now, and I'd like to contribute to something like this without having to sharpen up my rusty coding skills too suddenly.
EDIT: One last edit - also wanted to add in relation to the "we all start somewhere" comment - I couldn't agree more! Considering my background and interest in this topic, I will be happy to help anyone who is just starting out in this field as best I can. Just wanted to say this, in case anyone is hesitating to apply because of their lack of experience - I will do my best to help answer questions or point people to resources they might need (: