maint: Add ci workflow to check links

.github/lychee.toml

@@ -0,0 +1,8 @@
+include_fragments = true
+
+accept = ["200..=299", "429"]
+
+exclude  = [
+    "^http(s)?://localhost",
+    "^http(s)?://example.com"
+]

.github/workflows/check-links.yaml

@@ -0,0 +1,48 @@
+name: check-links
+on:
+  push:
+    branches: [main]
+  pull_request:
+  merge_group:
+    types: [checks_requested]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+permissions: read-all
+
+jobs:
+  changedfiles:
+    name: changed files
+    runs-on: ubuntu-latest
+    env:
+      PR_HEAD: ${{ github.event.pull_request.head.sha }}
+    outputs:
+      files: ${{ steps.changes.outputs.files }}
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+      - name: Get changed files
+        id: changes
+        run: |
+          files=$(git diff --name-only --diff-filter=ACMRTUXB $(git merge-base origin/main $PR_HEAD) $PR_HEAD | grep .md$ | xargs)
+          echo "files=$files" >> $GITHUB_OUTPUT
+  check-links:
+    runs-on: ubuntu-latest
+    needs: changedfiles
+    if: ${{needs.changedfiles.outputs.files}}
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+
+      - name: Link Checker
+        id: lychee
+        uses: lycheeverse/lychee-action@8646ba30535128ac92d33dfc9133794bfdd9b411 # v2.8.0
+        with:
+          args: "--verbose --no-progress ${{needs.changedfiles.outputs.files}} --config .github/lychee.toml"
+          failIfEmpty: false