docs: add some documentation dirs to the driver-api book

Those are subsystem docs, with a mix of kABI and user-faced
docs. While they're not split, keep the dirs where they are,
adding just a pointer to the main index.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/accounting/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ==========
 Accounting

Documentation/block/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 =====
 Block

Documentation/hid/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 =============================
 Human Interface Devices (HID)

Documentation/iio/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ==============
 Industrial I/O

Documentation/index.rst

@@ -92,6 +92,10 @@ needed).
 
    driver-api/index
    core-api/index
+   accounting/index
+   block/index
+   hid/index
+   iio/index
    leds/index
    media/index
    networking/index