improve: add basic docs about JSON-API and GraphQL

content/3.advanced-topics/30.jsonapi-graphql.md

@@ -1,3 +1,20 @@
 # JSON-API & GraphQL
 
-TODO: explain relationship. references
+When data needs to be queried or updated from the frontend, Drupal offers plenty of options. Lupus Decoupled Drupal's provided API may be combined with any of those:
+
+## JSON-API
+
+Drupal provides complete RESTful JSON-API module, which is included with Drupal core and works with all Drupal entities (and its access system) out-of-the-box. Following are the most relevant resources to get started:
+
+ * [Documentation](https://www.drupal.org/docs/core-modules-and-themes/core-modules/jsonapi-module
+   )
+ * [jsonapi-client npm package](https://www.npmjs.com/package/@drupal-api-client/json-api-client)
+ * [Json:API Extras module](https://www.drupal.org/project/jsonapi_extras) - Configure which entities are exposed
+ * [Simple OAuth](https://www.drupal.org/project/simple_oauth) - Authenticate requests via API token
+
+## GraphQL
+
+Alternatively, there are Drupal extension modules that add GraphQL support:
+
+ * [GraphQL module](https://www.drupal.org/project/graphql/)
+ * [GraphQL compose](https://www.drupal.org/project/graphql_compose)