Apart from what/why/why not: Something I started doing recently is to put URLs in the comments:

- URL of documentation for a complex feature

- URL with a dashboard with telemetry

- URL of a monitor which checks if the given feature, CI job, GitHub action etc. works correctly.

In a big project, figuring this stuff out is not trivial, requires a lot of searching with proper search terms and/or asking the proper knowledgeable person.

I find it weird that code is often so detached from everything non-code.