This is super cool. Thanks for posting, I am going to use it in my site!

A few suggestions if I may to make it a bit better and more portable.

1. Consider putting the friends YAML data into it own file in the _data directory instead of putting it in _config.yml. _config.yml is for site configuration information, it is too easy to polute and overload _config.yml with stuff that is better placed elsewhere. Instead put it in a friends.yml file stored in your _data directory. The syntax of your code would be almost the same, you would just have to change references of site.friends to site.data.friends.

2. a minor nit, the YAML syntax you use for friends is valid but punctuation rich :-). instead you can just use, the more common syntax...

  - title: "Title1"
    href: "https://example.com/"
  - title: "Title2"
    href: "https://example.com/"
  - title: "Title3"
    href: "https://example.com/"
  - title: "Title4"
    href: "https://example.com/"

3. Put your code in its own file and put it in the _includes folder, maybe called friends.html. Then in sidebar.html just do an "include", like this
   {% include friends.html %}
   This way, you can keep your code mostly separate from the chirpy theme code, and when an update to chirpy is posted, instead of a bunch of lines to modify again, you only have one.

4. Put your outer div inside the {% if site.friends %} block. No need to generate a div if there are no friends.

5. Similar to 3, but for the CSS. Instead of putting it directly in jekyll-theme-chirpy.scss, put it in its own file then use a css include to bring it into the works. The include would go either in main.scss or jekyll-theme-chirpy.scss.

These changes will help make a good thing a bit better.