Add CI workflow to check for broken links

.github/.markdownlinkcheck.json

@@ -0,0 +1,19 @@
+{
+    "ignorePatterns": [
+        {
+            "pattern": "http://127.0.0.1:8000/"
+        },
+        {
+            "pattern": "https://cra0.net/public/bin-published/Titanfall_VPKTool3.4_Portable.zip"
+        },
+        {
+            "pattern": "https://crates.io/crates/rrplug"
+        },
+        {
+            "pattern": "https://www.wunderboy.org/valve-hl2source-sdk-tools/#vtf_shell"
+        },
+        {
+            "pattern": "/_static/corner_test_map.map"
+        }
+    ]
+}

.github/workflows/linkcheck.yml

@@ -0,0 +1,14 @@
+name: Linkcheck
+on: [push, pull_request]
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: md-linkcheck
+        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          use-quiet-mode: 'yes'
+          config-file: '.github/.markdownlinkcheck.json'