samples: usb: update USB samples root documentation

Add note about legacy samples and update USB samples root documentation.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>

Author: jfischer-no

samples/subsys/usb/legacy/legacy.rst

@@ -0,0 +1,12 @@
+.. zephyr:code-sample-category:: usb_legacy
+   :name: Legacy USB device support
+
+   These samples demonstrate the use of legacy USB device support in Zephyr RTOS.
+
+.. note::
+   These samples use a legacy :ref:`usb_device_stack` that is in the
+   deprecation process.
+
+.. zephyr:code-sample-listing::
+   :categories: usb_legacy
+   :live-search:

samples/subsys/usb/usb.rst

@@ -1,6 +1,17 @@
 .. zephyr:code-sample-category:: usb
-   :name: USB device support
-   :show-listing:
-   :glob: **/*
+   :name: USB
 
-   Samples that demonstrate :ref:`usb_device_stack`.
+   These samples demonstrate the use of USB support in Zephyr RTOS.
+
+Most of the USB support samples can be found under :zephyr_file:`samples/subsys/usb`
+in the Zephyr project tree. However, there are a few functions whose samples
+are located in different directories. These are:
+
+* Bluetooth HCI USB transport layer implementation is covered by
+  :zephyr:code-sample:`bluetooth_hci_usb`
+* USB CDC ECM and USB CDC NCM implementations are covered by the networking
+  samples :zephyr:code-sample:`zperf` or :zephyr:code-sample:`sockets-http-server`
+
+.. zephyr:code-sample-listing::
+   :categories: usb
+   :live-search: