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/ap/show.ts

@@ -10,6 +10,8 @@ import Resolver from '../../../../remote/activitypub/resolver';
 import { ApiError } from '../../error';
 
 export const meta = {
+	tags: ['federation'],
+
 	desc: {
 		'ja-JP': 'URIを指定してActivityPubオブジェクトを参照します。'
 	},