Add the mermaid overview from the dev guide

docs/index.md

@@ -10,4 +10,30 @@ gateway and also is a dependency of every other API bundle.
 * It configures all dependencies to our needs (api-platform, symfony, etc.)
 * and more ...
 
+
+# Overview
+
 A minimal working relay API gateway consists of the core bundle and an auth bundle.
+
+```mermaid
+graph TD
+    style core_bundle fill:#606096,color:#fff
+
+    subgraph API Gateway
+        api(("API"))
+        auth_bundle("Auth Bundle")
+        core_bundle("Core Bundle")
+        core_bundle --> auth_bundle
+        api --> core_bundle
+    end
+
+    click auth_bundle "./#auth-bundle"
+    click user_session "./#usersession"
+```
+
+### Auth Bundle
+
+The auth bundle takes care of user authentication and communicates with an OIDC
+server, for example [Keycloak](https://www.keycloak.org). It creates the Symfony
+user object and converts OAuth2 scopes to Symfony user roles used for
+authorization.