feat: Enhance SEO and accessibility for website

- Updated title and meta tags in index.html for better search engine optimization.
- Added Open Graph and Twitter Card metadata for improved social media sharing.
- Included structured data for the game server in index.html.
- Improved accessibility with a skip link to main content in index.html.
- Enhanced player stats page (stats.html) with additional meta tags and structured data.
- Created robots.txt to manage web crawler access and included sitemap reference.
- Added sitemap.xml with URLs for the homepage and player stats page.
- Removed pigwei.html as it is no longer needed.

robots.txt

@@ -0,0 +1,27 @@
+# robots.txt for 白鹿原 Minecraft 服务器
+
+User-agent: *
+Allow: /
+Disallow: /stats/*.json
+
+# Sitemap
+Sitemap: https://mcpure.lunadeer.cn/sitemap.xml
+
+# 主要搜索引擎爬虫
+User-agent: Googlebot
+Allow: /
+
+User-agent: Baiduspider
+Allow: /
+
+User-agent: Bingbot
+Allow: /
+
+User-agent: Sogou web spider
+Allow: /
+
+User-agent: 360Spider
+Allow: /
+
+# 爬取频率建议（仅供参考）
+Crawl-delay: 1