Why does this file have an extension of .rst? What does that even mean for the fileformat?

https://en.wikipedia.org/wiki/ReStructuredText

This format really took off in the Python community in the 2000's for documentation. The Linux kernel has used it for documentation as well for a while now.

reStructuredText. Just like you have .md files everywhere.

Everyone missed a great opportunity to lie to you and tell you that the Linux kernel now requires you to program in rust.