samples/subsys/fs: move existing test into fat_fs

The sample is specific to the SDHC/fat_fs API.  Move it down to make
room for other file systems.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

samples/subsys/fs/fat_fs/README.rst

@@ -23,7 +23,7 @@ both the ``nrf52840_blip.overlay`` and the ``dts_fixup.h`` for nrf52840_blip
 to work:
 
 .. zephyr-app-commands::
-   :zephyr-app: samples/subsys/fs
+   :zephyr-app: samples/subsys/fs/fat_fs
    :board: nrf52840_blip
    :goals: build
    :compact:

samples/subsys/fs/fat_fs/sample.yaml

@@ -1,7 +1,7 @@
 sample:
   name: Fat filesystem sample
 tests:
-  sample.subsys.fs:
+  sample.subsys.fs.fat_fs:
     build_only: true
     platform_whitelist: nrf52840_blip olimexino_stm32 mimxrt1050_evk
     tags: filesystem

samples/subsys/fs/fs.rst

@@ -0,0 +1,10 @@
+.. _fs-samples:
+
+FS Samples
+##########
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   **/*