build.zig: update docgen to modern build system API
it still writes the output to zig-cache/langref.html but now it does
that explicitly as a legacy step with the intention of having that
removed in the future. It also outputs the langref to the install
prefix.
857296a9f4Andrew Kelley committed on 3/15/2023, 5:48:14 PMparentf829f84