Add Ahrefs update

src/content/blog/discovered-not-indexed.mdx

@@ -80,3 +80,9 @@ When I initially tested this approach last week, I had 60 unindexed pages,
 whereas the number is now down to 56, so it seems to be working.
 
 I'll update this post if I find better ways to fix this problem.
+
+Updates:
+
+* [Ahrefs](https://ahrefs.com/blog/discovered-currently-not-indexed/) has a
+  good post on the topic. For [pigweed.dev](https://pigweed.dev) I suspect that
+  the issue is related to how we've configured redirects.