docfix: Replace Skydoc reference with Stardoc, as former is deprecated
I just followed the Skydoc link from and see that Skydoc is "heavily deprecated" in favour of Stardoc, so I'm not sure if Skydoc should still be referenced in the latest Bazel  documentation.
Closes
@@ -261,7 +261,7 @@ it to [ci.bazel.build](http://ci.bazel.build).
## Documentation
-See the [Skydoc documentation](https://github.com/bazelbuild/skydoc) for
+See the [Stardoc documentation](https://github.com/bazelbuild/stardoc) for
instructions on how to comment your rules so that documentation can be generated
automatically.
