ci: add linker workflow to check against dead links

.github/workflows/linker.yml

@@ -0,0 +1,21 @@
+name: linker
+
+on:
+  push:
+    branches: [main]
+
+  pull_request:
+    branches: [main]
+
+jobs:
+  link:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Link Checker
+        uses: lycheeverse/lychee-action@v1.2.0
+
+        with:
+          fail: true