Or if someone wants to explain a piece of code where do they usually write about it?
If you are given a chance to build such a platform what would be it like?
Depends on the project and what is needed, generally a README.md for the big picture and an /docs for the generated API docs, hosted on github pages & friends or an webserver hooked up to the CI.
Which is often enough for coders but we also have a lua-jit running on top of the C++ where we have a plugin/module system where people can create extra functionality but some of those are rather new to code. I've written the API docs in LDoc (the only functional lua doc generator that works with the same version as lua-jit's lua version) but there currently isn't a nice way to implement a more of an manual which would solve a lot of redundant questions.