doc: boards: move subpages under Board support

Moved the board definition docs under Board support.
Removed the separate section for Boards from the top level.
These definitions may be moved to sdk-zephyr in a future release,
so having a top section for them is not needed, as we might end
up with an empty section.
Additionally, a top section with two entries might've suggested that
only two boards are supported by the nRF Connect SDK.
Discussed with @b-gent.
Follow-up to #27423.

Signed-off-by: Grzegorz Ferenc <Grzegorz.Ferenc@nordicsemi.no>

Author: greg-fer

doc/nrf/app_dev/board_names.rst

@@ -237,6 +237,24 @@ The following boards are defined in the :file:`nrf/boards/nordic/` folder.
 |                   |            |                                                          | ``thingy91x/nrf5340/cpunet``                                              |
 +-------------------+------------+----------------------------------------------------------+---------------------------------------------------------------------------+
 
+.. _nrf_boards:
+
+Board documentation in sdk-nrf
+==============================
+
+Out of the boards defined in sdk-nrf, the following boards have dedicated definition pages:
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Board definition pages in sdk-nrf:
+   :glob:
+
+   ../boards/nordic/*/doc/index
+
+.. note::
+
+   Some of these board definitions may move to the sdk-zephyr repository in a future release.
+
 .. _shield_names_nrf:
 
 Shields included in sdk-nrf

doc/nrf/boards.rst

@@ -44,7 +44,6 @@ Varied reference designs
 
    gsg_guides
    installation
-   boards
    app_dev
    test_and_optimize
    security

doc/nrf/index.rst

@@ -692,3 +692,5 @@ Documentation
 Updated:
 
   * The :ref:`ug_nrf54h20_architecture_lifecycle` documentation page, expanding the description of the LCS states.
+  * The :ref:`app_boards` documentation page by moving the pages with board definitions under the :ref:`nrf_boards` section.
+    This update removes the Supported boards section from the top level of the |NCS| documentation.