Add llms.txt into the site's config

Author: HyukjinKwon

_config.yml

@@ -5,5 +5,7 @@ kramdown:
 permalink: none
 destination: site
 exclude: ['README.md', 'content', 'LICENSE', 'merge_pr.py', 'Gemfile', 'Gemfile.lock']
+include:
+  - llms.txt
 keep_files: ['docs', 'static']
 url: https://spark.apache.org