Check links using lychee (#319)

Recently, our check_links workflow was failing very often, apparently
GitHub has introduced new rate limits. I played around with
[lychee](https://github.com/lycheeverse/lychee-action), a tool
specifically made for link checking, which supports caching and even
understands local links in Markdown and RST files.

Author: urfeex

.github/helpers/check_urls.sh

@@ -0,0 +1,42 @@
+name: Check Links
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  check_links:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Restore lychee cache
+        id: restore-cache
+        uses: actions/cache/restore@v4
+        with:
+          path: .lycheecache
+          key: cache-lychee-${{ github.run_id }}-${{ github.run_attempt }}
+          restore-keys: cache-lychee-
+      - name: Link Checker
+        id: lychee
+        uses: lycheeverse/lychee-action@v2
+        with:
+          fail: true
+          args: >
+            --verbose
+            --no-progress
+            --cache
+            --cache-exclude-status '429, 403, 404, 502'
+            --max-cache-age 2d
+            --exclude '^http://192\.168\.56\.101'
+            --exclude 'kernel\.org\/pub\/linux\/kernel'
+            --exclude 'releases/download/v\$%7BURCAP_VERSION%7D/externalcontrol-\$%7BURCAP_VERSION%7D\.jar'
+            --max-concurrency 1
+            './**/*.md' './**/*.html' './**/*.rst' './**/*.cpp' './**/*.h' './**/*.py'
+      - name: Save lychee cache
+        uses: actions/cache/save@v4
+        if: always()
+        with:
+          path: .lycheecache
+          key: ${{ steps.restore-cache.outputs.cache-primary-key }}

.github/workflows/check_links.yml

@@ -126,14 +126,3 @@ jobs:
           fail_ci_if_error: true
           token: ${{ secrets.CODECOV_TOKEN }}
           flags: start_ursim
-
-  check_links:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - name: Check URLs
-        run: |
-          .github/helpers/check_urls.sh \
-            -d ".git build CMakeModules debian" \
-            -f "package.xml urcl_architecture.svg trajectory_interface.svg dataflow.graphml start_ursim.sh ursim_docker.rst" \
-            -p "vnc\.html opensource\.org\/licenses\/BSD-3-Clause kernel\.org\/pub\/linux\/kernel"