There are a few Jekyll plugins enabling embedding tweets in your Jekyll websites (⚠️ not GitHub Pages!), such as jekyll-twitter-plugin. However, these plugins are not available with GitHub Pages due to security reason as the Jekyll website is mentioned.
Even though you can’t use those plugins, you are still able to embed tweets in GitHub Pages. I could not find any websites mentioning how to embed tweets in GitHub Pages with Jekyll, so I leave a note here.
How to embed tweets in GitHub pages is just to get a HTML code block from a tweet that you want to embed on Twitter.com, and then add it to a post file. Since Markdown is converted to HTML, adding HTML code tags to a Markdown file is totally valid. The below is an example of my embedded tweet.
Check out my iOS game, High5! https://t.co/QZEKLg3G2i— Keita Ito (@keitaitok) August 26, 2014
Happy blogging with GitHub Pages and Jekyll :)