docs: gpio: add sysfs interface to the admin-guide

While this is stated as obsoleted, the sysfs interface described
there is still valid, and belongs to the admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/admin-guide/gpio/index.rst

@@ -0,0 +1,17 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+gpio
+====
+
+.. toctree::
+    :maxdepth: 1
+
+    sysfs
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`