Add linkinator to check for broken links.

Currently only checks for internal links because external links can break at any time. Also, there are hosts that block linkinator.

package.json

@@ -17,9 +17,11 @@
   "homepage": "https://docs.pi-hole.net/",
   "scripts": {
     "markdownlint": "markdownlint \"**/*.md\" -i \"**/node_modules/**\"",
-    "test": "npm run markdownlint"
+    "linkinator": "linkinator site --recurse --silent --skip \"^(?!http://localhost)\"",
+    "test": "npm run markdownlint && npm run linkinator"
   },
   "devDependencies": {
+    "linkinator": "^2.1.1",
     "markdownlint-cli": "^0.23.1"
   }
 }