docs: link Bichon to its open-source project

The archival-email page linked to https://anhonesthost.com/bichon/, which
does not exist. Point readers at the upstream open-source project instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/content/docs/whp/add-ons/archival-email.mdx

@@ -18,7 +18,7 @@ Archival email keeps a long-term, searchable copy of your mail **outside** the l
 - Compliance or policy requires you keep email long-term.
 - You want a recovery option for mail you accidentally delete from the live mailbox.
 
-It's powered by our [Bichon](https://anhonesthost.com/bichon/) archival service.
+It's powered by the open-source [Bichon](https://github.com/rustmailer/bichon) archival service.
 
 ## How it's different from backups