Earlier quoted context omitted.
It is statically generated. The reason to use React is so you have one language across the front-end rather than having some people using React on one site, and then people using Gatsby/Hugo for something else. Next.JS can do the same thing as Gatsby/Hugo but has more features and is in React.
But you don't even need to know what language your tool is written in if all you want is a static documentation site. This is a problem a lot of people have; they think in technology instead of actual problem solving. Just look at how many projects have been posted on here with a title like "$solved_problem... in Rust!" as if Rust makes everything better forever. It's marketing bullshit. It's self-gratification. It's…
Not if you want interactivity in certain parts of the doc site, such as what Stripe does with API keys. It's simply easier to add JS if the entire toolchain is JS.