x/pkgsite: render type aliases in a group

[masiulaniec]

godoc renders each type alias in a dedicated <h2>. Please consider rendering all type aliases in one shared <h2> called Type aliases analogously to how constants and variables are currently rendered. This way, a code refactoring that breaks up a large package by moving a class of type declarations into a dedicated package will reap the benefit of making the table of contents shorter and thus more navigable.

[agnivade]

@dmitshur @andybons