doc: Explain how to locally build the documentation

Add a document that provides instructions on how to build a rendered
version of the documentation using the Sphinx tool.

Change-Id: I57159ec867a3ba7e003e7c7cd6ac53fb190356f2
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index a956efb..42a0b5b 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -5,6 +5,7 @@
    :maxdepth: 1
 
    requirements
+   docs-build
    obtain
    build
    build-options
@@ -16,6 +17,6 @@
 
 --------------
 
-*Copyright (c) 2019, Arm Limited. All rights reserved.*
+*Copyright (c) 2019-2020, Arm Limited. All rights reserved.*
 
 .. _Trusted Firmware-A (TF-A): https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git