Add a step to generate documentation to `build.zig` which `zig init` generates

[sorairolake]

Zig can generate documentation from doc comments.

For the library package, I think it would be useful to have a step in zig build to generate documentation. But there is no such step for build.zig which zig init generates.

So I think it would be useful to have a step like the following in build.zig which zig init generates:

zig/build.zig

Lines 51 to 68 in bf6ee7c

	const autodoc_test = b.addObject(.{
	.name = "std",
	.zig_lib_dir = b.path("lib"),
	.root_module = b.createModule(.{
	.root_source_file = b.path("lib/std/std.zig"),
	.target = target,
	.optimize = .Debug,
	}),
	});
	const install_std_docs = b.addInstallDirectory(.{
	.source_dir = autodoc_test.getEmittedDocs(),
	.install_dir = .prefix,
	.install_subdir = "doc/std",
	});
	//if (enable_tidy) install_std_docs.step.dependOn(check_autodocs);
	if (std_docs) {
	b.getInstallStep().dependOn(&install_std_docs.step);
	}

For those unfamiliar with the Zig build system, I think it is easier to edit a template rather than writing such a step from scratch.