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/federation/show-instance.ts

@@ -3,6 +3,8 @@ import define from '../../define';
 import Instance from '../../../../models/instance';
 
 export const meta = {
+	tags: ['federation'],
+
 	requireCredential: false,
 
 	params: {