docs(graphql): many-to-many relationship (#112)

docs/guide/graphql/relationships/index.md

@@ -6,8 +6,63 @@ title: Relationships
 
 ---
 
-// TODO
+The AnimeThemes API implements relationships for most types, previously known as allowed included in the REST API.
+
+// TODO: Waiting API refactor: pagination everywhere.
 
 ## Many-to-Many
 
-// TODO
+A many-to-many relationship has a `edges` list that contains edge objects.
+
+The edge object contains the pivot fields of the relationship and a `node` object that references the related type.
+
+Both fields `createdAt` and `updatedAt` are available for every many-to-many relationship.
+
+The following query
+```graphql
+query {
+    animes {
+        data {
+            name
+            resources {
+                edges {
+                    node {
+                        link
+                    }
+                    as
+                    createdAt
+                    updatedAt
+                }
+            }
+        }
+    }
+}
+```
+will return the JSON:
+```json
+{
+    "data": {
+        "animes": {
+            "data": [
+                {
+                    "name": ".hack//Liminality",
+                    "resources": {
+                        "edges": [
+                            {
+                                "node": {
+                                    "link": "https://myanimelist.net/anime/299"
+                                },
+                                "as": null,
+                                "createdAt": "2021-03-27T00:46:24+00:00",
+                                "updatedAt": "2021-03-27T00:46:24+00:00"
+                            }
+                            ...
+                        ]
+                    }
+                }
+                ...
+            ]
+        }
+    }
+}
+```