ci: add link checker

Signed-off-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>

.github/workflows/links.yml

@@ -0,0 +1,28 @@
+name: check links
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  checklinks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Restore lychee cache
+        uses: actions/cache@v3
+        with:
+          path: .lycheecache
+          key: cache-lychee-${{ github.sha }}
+          restore-keys: cache-lychee-
+
+      - name: Check links
+        uses: lycheeverse/lychee-action@v1
+        with:
+          args: "--cache --max-cache-age 1d ."
+          fail: true