x/pkgsite: better discoverability for extension interfaces

[josharian]

I was reading https://pkg.go.dev/io/fs, trying to find all the extension interfaces available for fs.FS. I had to scan the entire package API. It would have been even harder had the package not used a consistent naming convention.

Now that the standard library is using it, I suspect we'll see increasing uptake of the extension pattern. It'd be nice if pkg.go.dev could recognize it and surface extension interfaces, either through different grouping or through an explicit list at the base interface type.