mkdocs-material/docs/schema/plugins/external/section-index.json

{
  "$schema": "https://json-schema.org/draft-07/schema",
  "title": "Add index section pages to navigation",
  "oneOf": [
    {
      "markdownDescription": "https://github.com/oprypin/mkdocs-section-index",
      "const": "section-index"
    },
    {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "section-index": {}
      }
    }
  ]
}