docs: Standardize release navigation headers across Debian guides

Implements consistent pattern for release navigation following
the established Bullseye→Bookworm precedent:
- Current stable: Use navigation menu (no backward pointer)
- Old stable: Include "Newer release available" (forward pointer)

This pattern ensures new users find the current guide while
existing users on older releases are directed to upgrades.

Changes:
- Added "Newer release available" section to Bookworm guide
- Trixie guide uses navigation menu for finding older releases

Signed-off-by: wommy <tom@technomad.media>

Author: wommy

docs/Getting Started/Debian/Debian Bookworm Root on ZFS.rst

@@ -9,6 +9,14 @@ Debian Bookworm Root on ZFS
 Overview
 --------
 
+Newer release available
+~~~~~~~~~~~~~~~~~~~~~~~
+
+- See :doc:`Debian Trixie Root on ZFS <./Debian Trixie Root on ZFS>` for
+  new installs. This guide continues to exist for reference for existing
+  installs that followed it.
+
+
 Caution
 ~~~~~~~