Add lychee link checker workflow

.github/workflows/linkcheck.yaml

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+name: Link check
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  schedule:
+    # Mondays 06:00 UTC, so link rot is caught even without PR activity
+    - cron: "0 6 * * 1"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  lychee:
+    name: lychee
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Check links
+        uses: lycheeverse/lychee-action@e7477775783ea5526144ba13e8db5eec57747ce8 # v2.9.0
+        with:
+          args: >-
+            --config lychee.toml
+            --no-progress
+            'doc/content/**/*.md'
+            'exampleSite/content/**/*.md'
+            'README.md'
+            'CHANGELOG.md'
+          fail: false
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

lychee.toml

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+# Configuration for lychee (https://lychee.cli.rs)
+# Run locally with: lychee --config lychee.toml 'doc/content/**/*.md' README.md
+
+# Don't fail on these; 403/429 are usually bot-blocking, not dead links
+accept = ["200..=299", "403", "429"]
+
+max_retries = 3
+retry_wait_time = 5
+timeout = 20
+max_concurrency = 16
+
+# Hugo template syntax in layouts is not a URL
+exclude = [
+  "^https?://fonts\\.gstatic\\.com",
+  "^https?://fonts\\.googleapis\\.com",
+  "^https?://cdn\\.jsdelivr\\.net",
+  "^https?://cdnjs\\.cloudflare\\.com",
+  "\\{\\{.*\\}\\}",
+  "^https?://localhost",
+  "^https?://127\\.0\\.0\\.1",
+  "^https?://example\\.(com|org)",
+]
+
+# Relative links can't resolve without a built site
+exclude_path = []
+include_fragments = false
+exclude_all_private = true
+
+# Use GitHub API for github.com links (avoids rate limits; needs GITHUB_TOKEN)
+github_token = ""