I believe this feature was opened up recently (last month) and ever since it has been a rage. People have been updating their GitHub profile with a special repository using a README.md.
I came to know about it from the tech community I am part of. I used the following video that explains about how to set it up: https://www.youtube.com/watch?v=-otyb0ngsa4. Later, I checked and found many posts sharing the similar thing. This feature has piggy-backed on a convention (of README.md file) that is familiar to majority of GitHub users.
I went ahead and tried out myself. It supports markdown and thus it makes it easy to have content that has nice visual affects (yes, images and gifs are allowed!). README contents are placed above pinned repositories and thus prominently visible.
After setup, looking at it, a profile-level README seems like a great idea. This is another good opportunity (right next to your code repository) to let everyone know about yourself and showcase some highlights you feel important.
I read we can keep it auto-updating (like with our recent blog entries) using GitHub Actions. I am going to try that next. Go ahead and try out for yourself.
I have created my own repo for everyone to add/share their profile samples with the world. Please contribute and raise PR: https://github.com/sandeep-mewara/github-profile-README-samples
I found the official detailed documentation about it here: Setting up and managing your GitHub profile
Keep exploring!