This PR adds a few stylistic improvements to the distill-style blog
posts:
- Update distill blog post example to showcase sidenotes as an
alternative to styles in PR #3077
- Showcase equations in sidenotes (closes#1242)
- Add `overrides.js` to match footnotes and citation styles with global
theme styling
- Add "related posts" sections if enabled to distill style posts
- Add "giscus" section if enabled to distill style posts
https://github.com/user-attachments/assets/b19a9cd9-779d-4d89-a4fb-eb14fb384480
