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/web/index.ts

@@ -21,6 +21,7 @@ import getNoteSummary from '../../misc/get-note-summary';
 import fetchMeta from '../../misc/fetch-meta';
 import Emoji from '../../models/emoji';
 import * as pkg from '../../../package.json';
+import { genOpenapiSpec } from '../api/gen-openapi-spec';
 
 const client = `${__dirname}/../../client/`;
 
@@ -83,10 +84,19 @@ router.get('/manifest.json', async ctx => {
 
 // Docs
 router.use('/docs', docs.routes());
+router.get('/api-doc', async ctx => {
+	await send(ctx as any, '/assets/redoc.html', {
+		root: client
+	});
+});
 
 // URL preview endpoint
 router.get('/url', require('./url-preview'));
 
+router.get('/api.json', async ctx => {
+	ctx.body = genOpenapiSpec();
+});
+
 const getFeed = async (acct: string) => {
 	const { username, host } = parseAcct(acct);
 	const user = await User.findOne({