doc: Add guide for building the docs locally

This new page contains instructions for doing a local
build of the documentation, plus information on the environment
setup that needs to be done beforehand.

Change-Id: If563145ab40639cabbe25d0f62759981a33692c6
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index 23608f8..07e3753 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -7,6 +7,7 @@
    :numbered:
 
    user-guide
+   docs-build
    image-terminology
    porting-guide
    psci-lib-integration-guide