That depends largely on what you want to edit. Websites might be static pages, generated from some simpler markup language or hosted on a form of CMS. Documentation is often generated from source code comments to some extent.
If you can't easily figure out where it is stored and how it is generated you should probably ask along with your inquiry whether the maintainers would be interested in contributions in that area.
If you can't easily figure out where it is stored and how it is generated you should probably ask along with your inquiry whether the maintainers would be interested in contributions in that area.