ci: Introduce lychee to check links

Use `lychee` to check availability of links in cloud-hypervisor.

The urls explictly excluded in config file are manually checked.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

.github/workflows/lychee.yaml

@@ -0,0 +1,16 @@
+name: Link Check (lychee)
+on:
+  pull_request
+
+jobs:
+  link_check:
+    name: Link Check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Code checkout
+        uses: actions/checkout@v4
+
+      - name: Link Availability Check
+        uses: lycheeverse/lychee-action@master
+        with:
+          args: --verbose --config .lychee.toml .

.lychee.toml

@@ -0,0 +1,14 @@
+verbose = "info"
+
+exclude = [
+    # Availability of links below should be manually verified.
+    # Page for intel SGX support, returns 403 while querying.
+    '^https://www.intel.com/content/www/us/en/developer/tools/software-guard-extensions/linux-overview.html',
+    # Page for intel TDX support, returns 403 while querying.
+    '^https://www.intel.com/content/www/us/en/developer/tools/trust-domain-extensions/overview.html',
+    # Page for TPM, returns 403 while querying.
+    '^https://trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2p0-v1p05p_r14_pub.pdf',
+
+    # GitHub user smibarber referenced in `CREDITS.md` no longer exist
+    '^https://github.com/smibarber',
+]