Jekyll custom serving and .html extension

I’ve gotten around to serving jekyll myself outside of github.io. (At my work we have an internal docs serving site so I got the doc generation and deployment in our own CI, it’s pretty cool and I’m excited about it but I digress.) In order to serve these, I run a bundle exec jekyll build command and copy these over to an s3 bucket. When I then interact with the served site the .html is omitted from the links in the sidebar. This causes errors unless I manually add back the .html in the sidebar. But I’m curious as to why this doesn’t seem to be a problem with fastai-served pages.

https://dev.fast.ai/optimizer for example does not have a .html extension and works just fine.

Does anyone here know how I might achieve the same behavior in my site where the .html is not necessary?

Jekyll Builds should generate index.html

Example: https://github.com/fastai/fastpages/tree/gh-pages