website: update docusaurus

Currently, we cannot deploy the website because trying to run Docusaurus
on a version of Node newer than v16 causes SSL errors. This commit
updates docusaurus and adds a newly required slug field to the
front-matter of the introduction page.

Node > 16 is required because older base images in Netlify have been
deprecated and installing an old version of node is very slow.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

website/docs/README

@@ -1,5 +1,6 @@
 ---
 id: introduction
+slug: /introduction
 title: Introduction
 hide_title: true
 custom_edit_url: https://github.com/squat/kilo/edit/master/README.md

website/package.json

@@ -9,8 +9,8 @@
     "deploy": "docusaurus deploy"
   },
   "dependencies": {
-    "@docusaurus/core": "^2.0.0-beta.3",
-    "@docusaurus/preset-classic": "^2.0.0-beta.3",
+    "@docusaurus/core": "^2.2.0",
+    "@docusaurus/preset-classic": "^2.2.0",
     "classnames": "^2.3.1",
     "react": "^17.0.2",
     "react-dom": "^17.0.2"