This summer Tyler migrated the X-Plane scenery SDK documentation to X-Plane’s main wiki. I just put in a series of redirects, so that the old URLs for the top level pages will point to the various wiki sub-categories. The wiki contains the most recent information now, as well as up-to-date download links, etc.
At some point I will try to replace the individual scenery documents (part of the library.php script) with redirects to the appropriate wiki pages; until then I will leave the old site in place.