docs: Add 404 page and finalize unified documentation site

Co-authored-by: shenxianpeng <3353385+shenxianpeng@users.noreply.github.com>

Author: Copilot

404.md

@@ -0,0 +1,29 @@
+---
+layout: default
+title: Page Not Found
+permalink: /404.html
+---
+
+# Page Not Found
+
+Sorry, the page you're looking for doesn't exist.
+
+## What you can do:
+
+- **Go to the [Home Page](/)** to start over
+- **Check our [Quick Start Guide](quick-start/)** to get started
+- **Browse [Commit Check Tool Documentation](commit-check/)**
+- **Browse [GitHub Action Documentation](commit-check-action/)**
+
+## Common Links:
+
+- [Configuration Guide](commit-check/configuration/)
+- [CLI Arguments](commit-check/cli-args/)
+- [Troubleshooting](commit-check-action/troubleshooting/)
+
+## Still need help?
+
+- [Search GitHub Issues](https://github.com/commit-check/commit-check/issues)
+- [Join GitHub Discussions](https://github.com/commit-check/commit-check/discussions)
+
+[← Back to Home](/)