When you build a website which involves a lot of content that is highly varying in terms of structure and is being constantly created or updated, it's necessary to give the content creators good tools to work with (editing YAML files is not as much fun as some might think). These tools are normally called content management systems.
The architecture of a traditional CMS — say, Wordpress or Drupal — is incompatible with the JAMstack approach. These CMSes were built with very different ideas about content management in the first place: server- and database- centric and highly coupled. Also, these CMSes tend to produce all your front-end code as well, and that's not something you want to have. Instead, you need a simple instrument to create the content which can later be integrated into the JAMstack.
An alternative approach would be to use a CMS that delivers the content via an API. The editors would create and edit content in a very familiar fashion, whereas as a developer you would simply make an API call every time you need to insert content in your pages. This is a win-win for everyone, and this approach allows building fully custom web experiences from scratch, not being limited by the CMS, as you choose your stack yourself.
This philosophy plays perfectly well with Contentful — an API-centric, developer-friendly content management platform. It comes with an API to fetch the content and put it into your sites and apps. The content is of course formatted in Markdown (hello, JAMstack), and there's a web app where editors create it. We've built a reliable content delivery infrastructure behind it, and there are many SDKs for all the major programming languages, which greatly simplifies building projects with Contentful.
Take a look at our website to learn more about the product. And, as Mathias Biilmann puts it: spread the JAM!