samples: boards: Merge nrf91/ with nrf52/ and rename it to nrf/

The following samples:
- boards/nrf91/nrfx
- boards/nrf52/power_mgr
are actually not specific to nRF91 and nRF52 Series SoCs, respectively,
hence the current naming of the above board/ subdirectories may be
misleading. Thus, use nrf/ directory for holding all nRF board specific
samples that apply to several boards.

Paths to the moved samples that are mentioned in their documentation
are also updated accordingly.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>

samples/boards/nrf/mesh/onoff-app/README.rst

@@ -43,13 +43,13 @@ likely also run on the nrf52_pca10040 board.
 Building and Running
 ********************
 
-This sample can be found under :zephyr_file:`samples/boards/nrf52/mesh/onoff-app` in the
+This sample can be found under :zephyr_file:`samples/boards/nrf/mesh/onoff-app` in the
 Zephyr tree.
 
 The following commands build the application.
 
 .. zephyr-app-commands::
-   :zephyr-app: samples/boards/nrf52/mesh/onoff-app
+   :zephyr-app: samples/boards/nrf/mesh/onoff-app
    :board: nrf52840_pca10056
    :goals: build flash
    :compact:

samples/boards/nrf/mesh/onoff_level_lighting_vnd_app/README.rst

@@ -60,13 +60,13 @@ likely also run on the nrf52_pca10040 board.
 
 Building and Running
 ********************
-This sample can be found under :zephyr_file:`samples/boards/nrf52/mesh/onoff_level_lighting_vnd_app` in the
+This sample can be found under :zephyr_file:`samples/boards/nrf/mesh/onoff_level_lighting_vnd_app` in the
 Zephyr tree.
 
 The following commands build the application.
 
 .. zephyr-app-commands::
-   :zephyr-app: samples/boards/nrf52/mesh/onoff_level_lighting_vnd_app
+   :zephyr-app: samples/boards/nrf/mesh/onoff_level_lighting_vnd_app
    :board: nrf52840_pca10056
    :goals: build flash
    :compact:

samples/boards/nrf/nrfx/README.rst

@@ -28,12 +28,12 @@ This sample has been tested on the NordicSemiconductor nRF9160 DK (nrf9160_pca10
 Building and Running
 ********************
 
-The code can be found in :zephyr_file:`samples/boards/nrf91/nrfx`.
+The code can be found in :zephyr_file:`samples/boards/nrf/nrfx`.
 
 To build and flash the application:
 
 .. zephyr-app-commands::
-   :zephyr-app: samples/boards/nrf91/nrfx
+   :zephyr-app: samples/boards/nrf/nrfx
    :board: nrf9160_pca10090
    :goals: build flash
    :compact:

samples/boards/nrf/power_mgr/README.rst

@@ -39,7 +39,7 @@ Building, Flashing and Running
 ******************************
 
 .. zephyr-app-commands::
-   :zephyr-app: samples/boards/nrf52/power_mgr
+   :zephyr-app: samples/boards/nrf/power_mgr
    :board: nrf52_pca10040
    :goals: build flash
    :compact: