docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/index.rst

@@ -101,6 +101,7 @@ needed).
    filesystems/index
    vm/index
    bpf/index
+   usb/index
    misc-devices/index
 
 Architecture-specific documentation

Documentation/index.rst.rej

@@ -0,0 +1,10 @@
+--- Documentation/index.rst
++++ Documentation/index.rst
+@@ -103,6 +103,7 @@ needed).
+    vm/index
+    bpf/index
+    PCI/index
++   usb/index
+    misc-devices/index
+ 
+ Architecture-specific documentation

Documentation/usb/gadget-testing.rst

@@ -254,7 +254,7 @@ Device:
 - connect the gadget to a host, preferably not the one used
   to control the gadget
 - run a program which writes to /dev/hidg<N>, e.g.
-  a userspace program found in Documentation/usb/gadget_hid.txt::
+  a userspace program found in Documentation/usb/gadget_hid.rst::
 
 	$ ./hid_gadget_test /dev/hidg0 keyboard
 
@@ -886,7 +886,7 @@ host::
 	# cat /dev/usb/lp0
 
 More advanced testing can be done with the prn_example
-described in Documentation/usb/gadget_printer.txt.
+described in Documentation/usb/gadget_printer.rst.
 
 
 20. UAC1 function (virtual ALSA card, using u_audio API)

Documentation/usb/index.rst

@@ -0,0 +1,39 @@
+===========
+USB support
+===========
+
+.. toctree::
+    :maxdepth: 1
+
+    acm
+    authorization
+    chipidea
+    dwc3
+    ehci
+    functionfs
+    gadget_configfs
+    gadget_hid
+    gadget_multi
+    gadget_printer
+    gadget_serial
+    gadget-testing
+    iuu_phoenix
+    mass-storage
+    misc_usbsevseg
+    mtouchusb
+    ohci
+    rio
+    usbip_protocol
+    usbmon
+    usb-serial
+    wusb-design-overview
+
+    usb-help
+    text_files
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`