doc: extensions: add kconfig directive and role support

Add a new extension to support the :kconfig: role and .. kconfig::
directive. This removes the need of using :option:.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/_extensions/zephyr/kconfig-role.py

@@ -0,0 +1,27 @@
+"""
+Kconfig role and directive
+##########################
+
+Copyright (c) 2021 Nordic Semiconductor ASA
+SPDX-License-Identifier: Apache-2.0
+
+Introduction
+============
+
+A Sphinx extension that provides a role and directive for Kconfig content. The
+``:kconfig:`` role can be used inline to generate references to the page
+containing documentation for any Kconfig option. The ``.. kconfig::`` directive
+is automatically added to the Kconfig generated pages.
+
+Copyright (c) 2021 Nordic Semiconductor ASA
+SPDX-License-Identifier: Apache-2.0
+"""
+
+
+def setup(app):
+    app.add_crossref_type("kconfig", "kconfig")
+
+    return {
+        "parallel_read_safe": True,
+        "parallel_write_safe": True,
+    }