Generate a sitemap for docs.bazel.build

Closes #10983.

PiperOrigin-RevId: 301686893
diff --git a/scripts/docs/Gemfile b/scripts/docs/Gemfile
index 9f812cc..6512903 100644
--- a/scripts/docs/Gemfile
+++ b/scripts/docs/Gemfile
@@ -1,5 +1,6 @@
-gem 'jekyll', '3.8.6'
+gem 'jekyll', '~> 3.8.6'
 gem 'jekyll-paginate', '~> 1.0'
 gem 'pygments.rb', '~> 0.6.0'
 gem 'redcarpet', '~> 3.2', '>= 3.2.3'
 gem 'jekyll-toc', '~> 0.13.1'
+gem 'jekyll-sitemap', '~> 1.4.0'