fix: add sitemap

Author: qiwihui

Gemfile.lock

@@ -62,6 +62,8 @@ GEM
       sass (~> 3.4)
     jekyll-seo-tag (2.6.1)
       jekyll (>= 3.3, < 5.0)
+    jekyll-sitemap (1.4.0)
+      jekyll (>= 3.7, < 5.0)
     jekyll-target-blank (2.0.0)
       jekyll (>= 3.0, < 5.0)
       nokogiri (~> 1.10)
@@ -130,6 +132,7 @@ DEPENDENCIES
   html-proofer (~> 3.0)
   jekyll-feed
   jekyll-seo-tag
+  jekyll-sitemap
   jekyll-theme-cayman-blog!
   jemoji
   rubocop (~> 0.50)