Magnolia Headless Documentation is a separate website that consolidates the most relevant information for headless projects. It is a subset of the full documentation.
Are you starting a headless project? Read the Headless Documentation.
A key reason for the popularity of Headless CMS is the ease of use for front-end developers who consume content from the system. They are interested in a small subset of the Magnolia capability mostly because they will not deeply customize the system using Java. For them, the full documentation is not totally relevant to their needs.
On the other hand, some developers need comprehensive documentation of all aspects of Magnolia. That is why we have two documentation websites for these two audiences: