migrate langref documentation generation to the build system

doc/langref/test_aligned_struct_fields.zig

@@ -0,0 +1,16 @@
+const std = @import("std");
+const expectEqual = std.testing.expectEqual;
+
+test "aligned struct fields" {
+    const S = struct {
+        a: u32 align(2),
+        b: u32 align(64),
+    };
+    var foo = S{ .a = 1, .b = 2 };
+
+    try expectEqual(64, @alignOf(S));
+    try expectEqual(*align(2) u32, @TypeOf(&foo.a));
+    try expectEqual(*align(64) u32, @TypeOf(&foo.b));
+}
+
+// test