Anyone can be a mentor to someone else, irrelevant of a formal structure. We all can help in areas another needs it. This goes beyond your work team, too. You may be a mentor to someone you know online, met at a convention or meetup, or through a formal program.
Knowing you have the experience or skills to help and wanting to help them is only the beginning. Knowing how to help them is what will set you apart as a good mentor.
Getting to work on a new web development project is always exciting! But the excitement falls as soon I open the repository and see an empty README with nothing but the project title — if even that. Now instead of diving into building new features and squashing bugs, I'm spending hours trying to get started. I have to take time from other developers to ask why the build won't start, how to integrate with their custom services, where something should go in their code architecture, and so forth. Instead of spending my hours building and testing, I'm forced to wrestle information from multiple people if I can — some who aren't even available anymore and left the company.
Those are the good cases, too. Sometimes the original development team has long gone, there's no documentation, and no one to ask. In instances such as that, hours soar every time an unknown is encountered.
The worst part? All of these problems could have been avoided with the help of proper documentation.