Add doc

src/docs/api/entities/drive-folder.yaml

+name: "DriveFolder"
+
+desc:
+  ja: "ドライブのフォルダを表します。"
+  en: "A folder of Drive."
+
+props:
+  id:
+    type: "id"
+    optional: false
+    desc:
+      ja: "フォルダID"
+      en: "The ID of this folder"
+
+  createdAt:
+    type: "date"
+    optional: false
+    desc:
+      ja: "作成日時"
+      en: "The created date of this folder"
+
+  userId:
+    type: "id(User)"
+    optional: false
+    desc:
+      ja: "所有者ID"
+      en: "The ID of the owner of this folder"
+
+  parentId:
+    type: "entity(DriveFolder)"
+    optional: false
+    desc:
+      ja: "親フォルダのID (ルートなら null)"
+      en: "The ID of parent folder"
+
+  name:
+    type: "string"
+    optional: false
+    desc:
+      ja: "フォルダ名"
+      en: "The name of this folder"