Using Includes - shippify/slate GitHub Wiki

If you want to break your index.md into multiple files, use includes!

Each entry in the includes list at the top of index.md refers to a file in the includes directory, like this:

includes:
  - api/databases

Will append the content of includes/api/_databases.md to the end of index.md. Note that the (required) underscore in the filename isn't in the list entry.