I created GitHub Pages for both my user account and my project (the repo I wanted to be indexed), as proposed here https://github.community/t5/How-to-use-Git-and-GitHub/My-GitHub-repositories-are-not-known-by-Google/td-p/12993.
So the resulting links look like https://username.github.io and https://username.github.io/reponame. As I am the "owner" of https://username.github.io, I added the site as a "property" and verified my ownership over the "property" using Google Search Console. Then, also in Google Search Console, I manually requested indexing for both https://username.github.io and https://username.github.io/reponame. This made the two pages appear in results of Google search. But my GitHub repository still did not appear in results of Goolgle Search, even though the link to it was present on both the indexed GitHub Pages.
But on the next day I discovered that my GitHub repo unexpectedly appeared in Google search results. So it looks like the whole idea with creating GitHub Pages worked out for me.
Though, I also added topics (i.e. tags), description and website at the header of the GitHub repository. Maybe, this was which did the trick:
