Sanity.io is the platform for structured content. It comes with an open-source editing environment called Sanity Studio that you can customize with JavaScript, and a real-time hosted data store. Sanity has generous included quotas – so getting started is free.
Get started from the command line:
npm i -g @sanity/cli && sanity init
Or go to sanity.io/create to get started with a headless Jamstack website deployed on Netlify in minutes: Gatsby Blog, Gatsby Portfolio, Next.js Landing Pages, Gridsome Blog, Nuxt.js Events page, Sapper Blog, Eleventy Blog.
Sanity Studio is an open source headless real-time CMS, that you can customize with JavaScript and React.
Find more headless content management systems.