[github] How to fix HTTP 404 on Github Pages?

Another variant of this error:

I set up my first Github page after a tutorial but gave the file readme.md a - from my perspective - more meaningful name: welcome.md.

That was a fatal mistake:

We’ll use your README file as the site’s index if you don’t have an index.md (or index.html), not dissimilar from when you browse to a repository on GitHub.

from Publishing with GitHub Pages, now as easy as 1, 2, 3

I was then able to access my website page using the published at link specified under Repository / Settings / GitHub Pages followed by welcome.html or shorter welcome.