feat(linux): clarify U-Boot Target Images subsection usage

Since the first subsection is build/boot related, while the
second subsection is technical information relating to binary
layouts, a user may be confused if the first subsection is
necessary for them to read at all (it is!) if no clarification
is provided.

Signed-off-by: Yiyoung Liu <y-liu20@ti.com>

Author: maybe-yiyi

source/linux/Foundational_Components/U-Boot/BG-Target-Images-K3.rst

@@ -4,6 +4,13 @@
 Target Images
 #############
 
+This page describes the boot images produced by U-Boot builds for K3-based SoCs.
+
+See the :ref:`k3-images` section for which files to copy to your SD card.
+The :ref:`k3-image-formats` section provides technical details on binary layouts for reference.
+
+.. _k3-images:
+
 ******
 Images
 ******