I can’t say for sure exactly what Stripe is using, it could be a custom built static site, or they could be using one a static site generator with a custom theme.

Some tools that you can use to build doc sites similar to stripes off the top of my head:

GitBook - https://www.gitbook.com/

Hexo - https://hexo.io/

Docute - https://v3.docute.org/#/

Gatsby - https://www.gatsbyjs.org/

VuePress - https://vuepress.vuejs.org/