Depends. At the very least, answers to quick questions and all the pointers to references you'd want. Anything more would probably vary with the person and what people want and how many alum are interested.
If you're OK with possible second hand advice, feel free to still reach out.
Sure. Why not. Just open a PR.
PR - Pull Request is how you contribute to this effort. Once you have an account on Github, you can click the fork button on the top-right, make whatever changes you want locally and then submit a pull request. Once submitted, that gives us maintainers the ability to review it and once approved it gets added the main site.
Feel free to correct it if it is trivial. If link is completely dead, then remove it. If you aren't sure, consider reaching out to the original poster (see git blame) and see if they can update it.
Open an Issue explaining your idea and the merits behind it and we can talk. Or if it is simple just implement it and open a PR.