chore(deps): update dependency vitepress to ^0.21.0 (#866)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

package.json

@@ -136,7 +136,7 @@
     "ts-jest": "^26.5.4",
     "typescript": "4.2.3",
     "typescript-eslint-language-service": "^5.0.0",
-    "vitepress": "^0.18.0",
+    "vitepress": "^0.21.0",
     "vue": "^3.2.4",
     "vue-i18n": "workspace:*",
     "yorkie": "^2.0.0"