samples: usb: update USB samples root documentation

.. 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:

.. 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: