Makefile: Build docs with make

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

Makefile

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Huawei Inc.
+
+.PHONY: all
+all: build
+
+.PHONY: clean
+clean:
+	rm -rf build
+
+build:
+	sphinx-build -W . build