Introduce OpenAPI specs (#4351)

* wip

* wip

* wip

* Update index.ts

* Update gen-openapi-spec.ts

* Update api.ja-JP.md

* Fix

* Improve doc

* Update gen-openapi-spec.ts

* Update redoc.html

* Improve doc

* Update gen-openapi-spec.ts

* Improve doc

* Update CHANGELOG.md

src/server/api/endpoints/users/show.ts

@@ -13,6 +13,8 @@ export const meta = {
 		'ja-JP': '指定したユーザーの情報を取得します。'
 	},
 
+	tags: ['users'],
+
 	requireCredential: false,
 
 	params: {
@@ -42,6 +44,10 @@ export const meta = {
 		}
 	},
 
+	res: {
+		type: 'User',
+	},
+
 	errors: {
 		failedToResolveRemoteUser: {
 			message: 'Failed to resolve remote user.',