Zig build step for emitting docs by calling `zig build docs`

docs_step.zig

const docs_step = b.step("docs", "Emit docs");
const docs_install = b.addInstallDirectory(.{
  .install_dir = .prefix,
  .install_subdir = "docs",
  .source_dir = exe.getEmittedDocs(),
});
docs_step.dependOn(&docs_install.step);